Bunuel
What least number must be subtracted from 1936 so that the remainder when divided by 9, 10, 15 in each case is 7 ?
(A) 39
(B) 44
(C) 51
(D) 129
(E) 141
To get the general case of different divisors but common remainder, all we need to do is
\(n = LCM (9, 10, 15) * a + 7 = 90a + 7\)
So a number such that remainder upon division by 9, 10 or 15 will be 7 in each case will be of the form: 7 more than a multiple of 90, so it will definitely end in 7.
We will get such a number after every 90 numbers i.e. 97, 187, 277 etc. Hence the smallest number that we need to subtract from 1936 will not be more than 90. Hence options (D) and (E) are out.
To get a 7 in the units digit, we must subtract a number ending in 9 from 1936. So the only possible option is 39.
Answer (A)If you want to verify:
If we were to subtract 39 (option (A)) from 1936, we would get 1897. Is this of the form 90a + 7
1897 = 1890 + 7 = 90*21 + 7
Hence 39 is the smallest number which should be subtracted from 1936 to obtain the desired result.
Check the following video on Division and Remainders to understand this logic:
https://youtu.be/A5abKfUBFSc