Question 1069375
Let n = number of nickels
and q = number of quarters

n + q = 42  (1)
5n + 25q = 550  (2)     ($5.50 expressed as cents)


(1) implies  q = 42-n, substitute "42-n" for "q" in (2):
         5n + 25(42-n) = 550
    
From here, I will leave it to you to combine terms and solve for n.  Once you have n you can plug into (1) to get q.