This is the final post in the age guessing series.
As with the previous post, this will mix the approximate judging of someone's age with math, but this version hides the math better. That's because it focuses on calendar dates!
In this approach, you have a spectator give you their birthday, without the year. You then have the spectator look up the day of the week on which they were born in a perpetual calendar, and tell you that day of the week (again, without telling you the year). You then look them over, and announce their exact age!
For example, let's say your spectator tells you they were born on June 16th. After looking up the day of the week they were born, they tell you it was a Friday. After looking them over briefly, you announce (correctly) that the person was born in 1978!
Basic conceptsThis is the most advanced age-guessing routine, and you'll need to master two other skills before learning this feat.
First, you'll need to have a good practiced ability to approximate someone's age, as taught in the Judging Appearances post. Second, you'll need a thorough mastery of the Day of the Week For Any Date feat, as taught here on Grey Matters. Especially important in this version of the feat will be an understanding of subtracting multiples of 7, an understanding of the basic formula, and the memorization of all 100 year keys from 0 to 99.
In the classic calendar formula, you add m (month key) + d (date key) + y (year key) = a (answer key for day of the week). In this feat, we're focusing on age, so we need to rework this formula to provide the year key as an answer. Subtracting d and m from both sides, we get a - d - m = y. In other words, we start with the key for the day of the week, subtract the key for the date, then subtract the key for the month, and we'll wind up with the key number for the year they were born.
Remember, though, that a, d, and m can all be numbers from 0 to 6, so you might wind up with problems such as 4-6-5. To avoid dealing with negative numbers, it's best to start by adding 14 to a, the day of the week key, right at the beginning. This will keep the day of the week key large enough to prevent negative numbers as answers. Since 14 is a multiple of 7, it won't change anything if you need to subtract multiples of 7 later on (I told you that would be important!). Modifying the formula to take this into consideration, and make the mental math easier, we have: (a + 14) - d - m = y.
Step by step1) Begin right as you select the person whose age you will determine. Use your age-approximation skills to determine the person's approximate age, and save this for later as your preliminary guess.
Example: Let's say our spectator looks to be in his mid-30s, so you make a preliminary guess of 35. You don't mention this guess out loud. If you're performing this in 2012, you work out that being 35 means that he would have been born in 1977. For now, just keep the year 1977 in the back of your mind.
2) Ask them to give you their birthday, but without the year. As you explain about looking up the day of the week on which they were born, you'll need to recall the month key, and reduce the date they give by subtracting the multiple of 7 lesser than or equal to the date you were given.
Example: Our spectator says they were born on June 16th. The key for June is 3 (as shown in the chart on this page), and 16, when subtracting the nearest multiple of 7, becomes 2 (This calculation is also known as 16 mod 7 = 2). Remember the numbers 3 (month key) and 2 (date key).
3) Have them look up the day of the week in which they were born, and announce that day of the week. This can be done using a perpetual calendar you bring, or by using an app or website on their mobile device. Mentally convert the date they give you into its key number (according to the day of week key chart here).
Example: They look up June 16th in a perpetual calendar, and announce that they were born on a Friday. The key number for Friday is 5.
4) Now that you've got all the numbers you need, plug them into the formula: (a + 14) - d - m = y.
Example: Since a (answer key for day of week)=5, d (date key)=2, and m (month key)=3, we work out (5+14)-2-3=19-2-3=17-3=14.
5) If they appear to have been born anytime in the 1900s, subtract 1 to compensate for the century (the reverse of adding 1 in the original feat).
Example: Since our spectator definitely appears to have been born in the 1900s (1977 was our preliminary guess), we work out 14-1=13.
6) At this point, if your mental running total is greater than 6, subtract the nearest multiple of 7 equal to or less than the total to get your final year key.
Example: Our running total is 13, and the nearest multiple of 7 equal to or less than the total is 7, so we subtract 13-7=6. 6 is the final year key in this example.
7) Recall your preliminary mental guess from step 1. Using your memorized list of year keys, ask if the corresponding year in the 2000s (the year 100 years later) has the same year key as the one you calculated.
Example: Our preliminary guess was 1977, so we think about 2077 (remember, we subtracted 1 to adjust the year to the 2000s, which we've already memorized), and recall that it has a year key of 5. The year key we're looking for is 6, so '77 is obviously not the correct year.
8) Try changing forwards or backwards by one year, and find the closest year with the correct key. While you're mentally searching for a year, you can pretend to be studying the person closely for signs of their age. This not only gives you more time for your mental search, but can potentially be very entertaining, as well.
Example: Since 1977 was wrong, yet very close, we move forward a year and try 1978. Recall that 2078 has a year key of 6, which is the year key we're looking for!
9) Once you've found the closest year with correct key, work out the age that would make them and announce that as your guess! Assuming you're correct, bow to thunderous applause!
Example: We worked out that 1978 (well, actually 2078, but in 2012, we can be sure they weren't born in 2078) has the correct key. Being born in 1978 means they'll turn 34 in 2012, so we make a guess of 34 out loud!
Even if you get their age wrong (hopefully by guessing too young, as people will always forgive that), you can still save the trick by pointing out that the age you guessed would've put their birthday on the correct day! This is still quite impressive, and implies a seemingly impossible knowledge of dates.
Unlike the original day of the week for any date feat, the emphasis here isn't on speed. As I mentioned in step 8, you can do your mental calculations while walking around the person and pretending to be examining them for signs of their age, which you've already secretly done before the trick even started.
As always, don't forget that age can be a touchy subject, and treated with caution. Explain at the beginning that you want someone who is willing to not only state their actual age, but have their age announced out loud before an audience, as well.
I hope you've enjoyed this series on age guessing. If you have any questions about any of the posts, please let me know in the comments, and I'll do my best to answer them.