The value of 40 coins of 10 p and 20 p is Rs. 5.50. The number of 20 p coins is
11
13
15
25
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An enterprising businessman earns an income of Re 1 on the first day of his business. On every subsequent day, he earns an income which is just double of that made on the previous day. On the 20th day of business, he earns an income of: