Question 847472
Let  n = # of nickels
Let  4n = # of 50-cent coins
Let  42-5n = # of ten-cent coins


.05 * n  + .50*4n + .10(42-5n) = 13.50

.05n + 2n + .42 - .5n = 13.50

1.55n = 9.3

n = 6

42-5n represents ten-cent coins  42-5(6) = 12 ten-cent coins <--- ANSWER

Verify:

6 nickles
24 50-cent coins
12 dimes
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42 coins

6*.05 + 24*.50 + 12*.10 =  .30 + 12 + 1.20 = 13.50