section A question

An office manager of Harris Plc wishes to minimise the cost of telephone calls made. 40% of calls in peak hours cost $1 each and the remainder of such calls cost $1.50 each. 30% of calls at other times cost $0.80 each, 50% of them cost $0.90 each, and 20% of them cost $1 each. This proportion cannot be varied, though the total number of calls made in peak hours and of calls made at other times can be. If X = the numbers of calls made each day in peak hours, and Y = the number of calls made each day at other times, the official manager’s objective is to:
A. Minimise 120X + 89Y
B. Minimise 120X + 90Y
C. Minimise 130X + 89Y
D. Minimise 130X + 90Y

sir could you please explain how to solve the following question. the answer is C.