It is nice to be back with you after the break. Let us play with some numbers and enjoy our learning experience. To start with I am going to recall some very easy methods using which we arrive at answers without doing calculations.
First, the nine times tables on your hand.
Put both your hands on the table with palms facing up. The left thumb is 1, the left index finger is 2 and so on up to 10. The right thumb is 10. To find the product of 9 and any number from 1 to 10:
Fold the finger which corresponds to that number. For example to find 9 x 4, fold finger number 4. Count the fingers on either side of the folded finger. You will have 3 on the left and 6 on the right of the folded finger. So the answer is 36.
To find 9 x 8, fold finger number 8. If you count the fingers on both sides you get 7 and 2. So the answer is 72.
Next let us recollect the concept of complements and use that in addition. To make addition easier we rewrite the number as the nearest multiple of 10, or the nearest zero ending number. For that we need to recollect complements: numbers that add up to 10. 1&9, 2&8, 3&7, 4&6, 5&5 are complements.
Say you have to add 58 and 72.
■ 58 is close to 60. It is 2 less than 60, the nearest zero ending number.
■ 72 is close to 70 and 2 more than 70.
■ Take away 2 from 72 and use it to make the 58 into 60.
■ Now our sum has changed to 60 + 70.
■ 60 + 70 = 130. That is the answer.
Even if more than two numbers are to be added, we can use the same method.
Example: 24 + 17 + 8 + 16 + 23 = ?
■ First look at the units digits of the numbers.
■ 24 and 16 can be combined since 4 and 6 are complements and mentally added: 20 (from 24) + 10 (from 16) + 10 (adding units digits 4 + 6)
■ We get 40. Keep this in mind.
■ Combine 17 and 23 as 10 (from 17) + 20 (from 23) + 10 (7 + 3) = 30.
■ We are left with 8 and the sum is 40 + 30 + 8 = 78.
These are for you to try:
7 + 8 + 3 + 9 + 2 + 5 = ?
12 + 14 + 18 + 16 + 9 = ?
24 + 33 + 46 + 19 + 11 + 17 = ?
51 + 34 + 44 + 16 + 19 + 26 + 7 = ?
Tip of the week: Practice makes perfect