a piggy bank contains 30 coins, all dimes and quarters worth 5.70. how many dimes and quarters are in the bank?

D+Q=30
.1D+.25Q=5.7 (m by 20) 
D+Q=30 
2D+5Q=114 

D+Q=30 (m by -2)
2D+5Q=114 
-2D-2Q=-60
2D+5Q=114 

adding... 
3Q=54
Q=18 
D=30-18=12 
12 dimes, 18 quarters