Thuto has some money in a coin box. He adds equal sums of money to the box every day. After the first day the box contains 35t, after the second day it contains 50t and after the third day it contains 65t. He continues to save for several more days.

1. aHow much money will be in the coin box after saving on the 4th day? [1]
2. bFind the expression, in terms of n, for the amount of money in the box after saving on the nth day. [2]
3. cHow much money will be in the coin box after saving on the 20th day? [1]

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