It's valuable to know how to represent remainders in algebraic terms. When m is divided by n, there is an integer quotient (in this case, 24), and the decimal part consists of the remainder divided by the denominator. For instance, when 4 is divided by 3, the quotient is 1 and the remainder is 1:
4/3 = 1 + 1/3
In general terms:
m/n = q + r/n
We can ignore the quotient in this problem: We know it's 24. The fractional part, however, is represented in two ways. First, it is equal to 0.2. Second, it is equivalent to r/n, or 12/n. We can solve for n by setting those two equal to each other:
0.2 = 12/n
2/10 = 12/n
2n = 120
n = 60, choice (B).