## Walter Gibson

Any time you are within shouting distance of a calendar, is will be possible to present an offhand feat to upset good mathematicians. The assistant takes a monthly sheet; you turn your back. He is told to mark off one day in each week. You now ask, "How many Sundays are checked?" "How many Mondays?" "How many Tuesdays?" etc. Immediately after this you name a number. The calendar man adds up the dates he has crossed and finds that you have correctly called the totalI

Our secret is well hidden. A calendar page is illustrated. Try always to use pages wherein 5 lines of dates appear, and Wed. represented by 5 date figures. However, those who use this neat problem at all will accustom themselves to the variations. It is only necessary to know the total of the 5 Wed. dates, in this case 80.

The subject crosses ONE day in EACH of the FIVE lines. More than 1 date can be checked off on the same DAY, as long as it's in a different WEEK. You carry 7 mental values. Sun is -3. Mon la -2. Tue is -1. Wed is 0. Thu is -«-1. Fri is +2. Sat is +3. Dates are crossed out in the illustration for example. When asked how many Sundays are checked, the reply is 1. You mentally say -3. There is no Monday checked so you repeat -3. One Tuesday. You say -1, and combine it with -3 to make -4. No Wednesday. Two Thursdays. +1 and +1 are combined with -4 which gives -2. Ho Friday. One Saturday. +3 is combined with -2 which gives a final total of +1. In your mind you add this to the key number of 80, and 81 is the total of the 5 dates crossed out.

Remember the key total of the five Wednesdays. This day is neutral. It's no matter how many such days are marked. Your mental figures don't change. The day values are 3-2-1-0-1-2-3. Before Wed. minus. After Wed. plus.

Sim lie hij 71m Tri Sel

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