Division by 9

“Most of us want to avoid 9 in almost all calculations. But again, we can make calculations easier by thinking of 9 as the difference of 10 and 1, (10 – 1). This fact is particularly useful in division by 9. Every 10 contains a 9 and a remainder of 1. So every multiple of 10 less than 90 will have a quotient and remainder equal to its tens digit.

            So        20 ÷ 9 = 2 r. 2

                        40 ÷ 9 = 4 r. 4

            And     70 ÷ 9 = 7 r. 7

“Extending this observation, we can readily obtain the quotient when small numbers are divided by 9. Take the case of 34. When divided by 9, the quotient is equal to the tens digit 3 and the remainder is equal to the sum of the tens and units digits, 3 + 4 = 7

            42 ÷ 9 = 4 r. 6                          71 ÷ 9 = 7 r. 8

            26 ÷ 9 = = 2 r. 8                       69 ÷ 9 = 6 r. 15

“In 15, we can still  get a 9 and a remainder of 6 so 69 ÷ 9 is 7 remainder 6. At this point we would like to stress that the following results are equivalent:

            69 ÷ 9 = 6 r. 15 = 7 r. 6 = 8 r. -3 but 7 r. 6 is the best form

“This technique can also be used for longer number. To divide longer numbers by 9, get the running sum of the digits of the number.” (from page 37, 25 Math Short Cuts which is available at MATH-Inic Philippines – Math Made Fun, Fast and Easy https://www.facebook.com/MATHInicPhils)

We can mentally answer our featured example 102003000 ÷ 9 by

1. noting that the dividend has 9 digits, then the answer must have 8 digits
2. get the running total of the digits of the dividend: the first digit of the answer is 1 while the second is 1 + 0 = 1. So the answer begins with 11 million…
3. the next digits are 1 + 2 = 3, 3 + 0 = 3 and 3 + 0 = 3: 333 thousand …
4. the last three digits of the answer are 3 + 3 = 6, 6 + 0 = 6 and 6 + 0 = 6: 666
5. And the remainder is 6 + 0 = 6.

This example does not require regrouping or any adjustment in the answer. Cases involving large digits in the dividend which may lead to double digit running sums and, hence regrouping, will be discussed in future posts.

This technique would be useful in polynomial division.

Practice exercises:

1. 52 ÷ 9 =
2. 71 ÷ 9 =
3. 123 ÷ 9 =
4. 142 ÷ 9 =
5. 2121 ÷ 9 =
6. 3113 ÷ 9 =
7. 11221 ÷ 9 =
8. 22111 ÷ 9 =
9. 300211 ÷ 9 =
10. 321001 ÷ 9 =

Answers to last week’s exercises:

Find the remainder when the results of the following operations are divided by 9.

1. 327 + 633 → 6
2. 989 – 543 → 5
3. 789 x 536 → 3
4. 22712 ÷ 334 → 5
5. 7788 + 8877 → 6
6. 8641 – 6421 → 6
7. 3468 x 5672 → 6
8. 24375 + 89731 → 4
9. 76534 – 18939 → 4
10. 77777 x 88888 → 5