Question

the least number which must be subtracted from 6709 must be divisible exatly by 9

Answer 1

To be divisible by 9 sum of all digits should be divisible by 9. subtract 4 from 22 will satisfy the condition. So subtract 4 from 6709 , which is 6705 which is perfectly divisible by 9. Hope it helps.

Answer 2

$\huge \colorbox{cyan}{ᴀɴsᴡᴇʀ}$

Sum of digits should be divisible by 9

6+0+7+8 =21

As no to be subtracted should make it divisible by 9,  Sum  of digits can be 18 

3 should be subracted.