Tina decides to fill a jar with coins. In the first hour she puts in 20 coins. During the next two hours she puts in 30 coins each time. During the fourth hour she puts in 40 coins. During the fifth hour her mother asks to borrow some money so she takes 20 coins out. How many coins are left after the fifth hour? Give me reasons, before answering the question
There are 20 coins during the first hour. 30 coins are added during the second and third hours and 40 coins are added during the fourth hour. By the fourth hour there are 20 + 30 + 30 + 40 = 120 coins. The number of coins after giving 20 to her mother is 120 - 20 = 100. So the final answer is 100.