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Question

Teacher lent out 11 rulers at the beginning of class. Collected 4 rulers in the middle of class. Gave 7 out at the end of class. He had 18 at the end of the day. How many rulers did he start with?

Answer 1

This is a classic case in which you have to work backwards.

 

He ended with 18.

The thing he did before that was to give out 7, so he must have had 25...so that when he gave out 7, it left him with 18.

 

But before that, he collected 4...so he must have had 21, collected 4 to get 25 and then gave out 7 to end with 18.

 

But before that, he gave out 11. So he must have had 32 so that when he gave out 11, he ended with 21, and then he collected 4 giving him 25 and then gave out 7 ending with 18.

 

So he started with 32.

 

Hope you could follow this working backwards logic.

Answer 2

r represents the number of rulers

 

r - 11 + 4 - 7 = 18

 

Can you simplify and solve for r?