Problem: What least number must be subtracted from 13601, so that the remainder is divisible by 87?

Solution:
To find the number that must be subtracted from 13601 so that the remainder is divisible by 87, we need to follow the below steps:

Step 1: Find the remainder when 13601 is divided by 87.
We can use the modulo operator (%) in programming or use long division to find the remainder. Using long division, we get:

```
156
------------
87 | 13601
- 87
------
730
-696
------
34
```
Therefore, the remainder when 13601 is divided by 87 is 34.

Step 2: Find the number to be subtracted from 13601.
Let x be the number to be subtracted from 13601. Then, we have:

13601 - x ≡ 0 (mod 87)

This means that x is the smallest positive integer that satisfies the above congruence. We can rewrite the above congruence as:

x ≡ 13601 (mod 87)

To solve for x, we can add or subtract multiples of 87 to the right-hand side until we get a number that is less than 87. We have:

13601 ≡ 34 (mod 87)
13601 - 34 ≡ 0 (mod 87)
13567 ≡ 0 (mod 87)

Therefore, the least number that must be subtracted from 13601 so that the remainder is divisible by 87 is 34.

Answer: The answer is (a) 23.