book Maths Puzzle Numerical Ability

I COLLECTED 4 TIMES AS MUCH MONEY AS DERRICK. WE COLLECTED A TOTAL OF $3900 HOW MUCH DID EACH OF US COLLECT. THIS ONE REALLY PUTS ME IN A STUMP HOLE THANKS FOR ANY HELP. SINCERLY KEVIN

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yearly average of 35.18% and another of 33.43%, these are my answers but Im not sure how to figure out the number of successful repairs each candidate would make on 50 calls?

pl
I am trying to work out a problem and I am not sure how to work it. The problem was given to my class out of the book for extra credit.
The problem is: 2x2-x-21.

It is 2xsquared subtract x subtract 21
Thank You
Brittany Miller