In a store, Arun has two options to buy a mobile phone, either he can pay the full amount of Rs. 2,500 or he may buy at an instalment scheme, i.e. Rs. 520 in down payment and four equal instalments, rate of interest being charged at 25% p.a. simple interest. Arun buys a mobile phone on the instalment scheme. What is the monthly instalment for Arun?