Group-replacement policy — comparing average cost per week

Suppose that there are 1000 components in operation. The (cumulative) number of failed replacements at the end of each week are: W1: 50, W2: 82, W3: 128, W4: 199. If the cost of replacing an individual failed component is Rs. 125 and the cost of group replacement of all 1000 components is Rs. 300, what is the best interval between group replacements?