It took marty 42 minutes to write a book report. then he spent 18 minutes correcting his report explain how you can use mental math to find out how long marty spent on his book report

You simply add both times in your head to find the total of how long Marty spent on his book report.

So as of now you have two numbers . You break down the problem and in this situation they are asking us to add both numbers .. in total Marty spent 42+18 = 60 mins in total on his book report.

The answer should be c.