A change purse contains 120 coins worth $10.They are all nickels and dimes.How many of each coins are there?

(1) n+d=120 <- number of coins
(2) 5n+10d=10(100) <- amount of money 


5*(1) gives us...
5n+5d=600 


(2) -  5 * (1) gives us...
10d-5d=400
5d=400
d=80 
80 dimes and 40 nickels