Vodafone charges a fixed rental of Rs. 450 per month. It allows 200 calls free per month. Each call is charged at 1.8 when the number of calls exceeds 200 per month and it charges Rs. 2 when the number of calls exceeds 400 per month and so on. A customer made 150 calls in June and 535 calls in July. By how much per cent the each call is cheaper in July than each call in June?