Question 380366
let f be the number of fifty cents coins, then there are (30-f) quarter, we shall have : 

f*0.5+(30-f)*.25 = (20-9.5) = 10.5$
 
f/2 + 30/4 - f/4 = 10.5
 
f/4 = 21/2 - 30/4 = (42 - 30)/4
 
f = 12 coins of fifty cents and 18 coins of a quarter $.