Sophie Knowles doesn’t expect that everyone will be able to unwind with arithmetic, but she feels that doing puzzles and mental arithmetic keeps you sharp, and improves your memory and your powers of observation. Here are some samples of puzzles and games that exercise your wits.
Browse in your bookstore and library, and online for more brainteasers and have some fun!
1. Why is 6 afraid of 7?
ANSWER: Because 7 8 9.
2. Try solving this classic riddle.
As I was going to St. Ives
I met a man with seven wives
Each wife had seven sacks
Each sack had seven cats
Each cat had seven kits
Kits, cats, sacks, wives
How many were going to St. Ives?
ANSWER: Only 1, the narrator. There’s no indication that the others were going to St. Ives.
Another interpretation, that all were going to St. Ives, requires a considerable amount of arithmetic:
1 narrator
1 man
7 wives
49 sacks (7 x 7)
343 cats (49 x 7)
2401 kittens (343 x 7)
Total = 2802
Multiplying a 2-digit number by 11
There are several shortcuts for this case. Here’s a two-step device for multiplying a 2-digit number by 11 without using paper.
1. Take the number (we’ll use 52) and imagine a space between the two digits:
5_2
2. Now add the two numbers together and put the sum in the middle:
5_(5+2)_2 = 572.
That’s it!
If the numbers in the middle add up to a 2-digit number, just “carry 1 over”-that is, insert the second number in the middle, and add 1 to the number on the left. For example, for 99 x 11, the steps are:
9_(9+9)_9
9_(18)_9
(9+1)_8_9
10_8_9
1089 is the answer!
Another way to multiply by 11 is to multiply the number by 10, then add the original number:
52 x 10 = 520
520 + 52 = 572
For the second problem, 99 x 11:
99 x 10 = 990
990 + 99 = 1089
The correct answer, again!
The doublet, attributed to Lewis Carroll, involves transforming one word into another by changing only one letter, with each intervening change being a word.
Example: Transform HEAD into TAIL.
ANSWER: (the letter in bold is the letter changed on the way to the final word):
HEAD
HEA L
T EAL
TE L L
T A LL
TAIL
Try turning WHEAT into BREAD!