George has 15 coins, all 50c pieces and 20c pieces. Of 20c pieces were 50c pieces and 50c pieces were 20c pieces, he would have $2.10 more. How much 50c pieces does he actually have?
If there are f 50c pieces and t 20c pieces, then we have
f+t = 15
20f + 50t = 50f + 20t + 210
that gives you
30t - 30f = 210
30(15-f) - 30f = 210
f = 4
check:
50*4 + 20*11 = 420
20*4 + 50*11 = 630 = 420+210