A wonder sum that is always the same.
Take any three digit number in which the first digit is larger than the last say 725. Reverse it making 527 and subtract the smaller from the larger, making 198. Now add the result to the same number reversed, that is 891. The answer is 1089 — and will be 1089 whatever number you start with.
Time you would take to count one billion.
If a person counted at the rate of 100 numbers a minute, and kept on counting for eight hours a day, five days a week, it would take a little over four weeks to count to 1 million and just over 80 years to reach one billion.
How can you tell the age of your friend?
Ask him to write down his age say 16. Then double it, making 32. Add one to it, making 33. Multiply it by 5, making 165. Now add 5 to it, making 170. Now multiply it by 10 making 1700. Then subtract 100 making 1600. Now ask him to tell the number. You delete the last two digits. The remaining number will be his age i.e. 16.
Describe a number game such that by manipulations answer is always 15
Think any number say 12, multiply it by 5 making 60. Add 25 to it, making 85, divide it by 5, making the quotient 17, subtract the number you started with 17-12 = 5. Multiply the remainder by 3 making it 15. So answer will always be 15.
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