Question 53591
From the answer you came up with, I think you meant that there was 150 nickles and dimes not 50.  So I changed your question, if I'm wrong let me know.
This is a pretty tough question for a third grader.  Your son must be pretty smart.
Let the number of dimes=x
Let the number of nickels=x+70
If we add the nickels and dimes together=150
Your problem to solve is:
x+x+70=150
(1+1)x+70=150
2x+70=150
2x+70-70=150-70
2x=80
2x/2=80/2
x=40
The number of dimes=x=40
The number of nickles=x+70=40+70=110