Question 148274
D + Q = 30 (30 Dimes and Quarters)
10D + 25Q = 600  ($6)
-------
Substitute for D (D=30-Q)
10(30-Q) + 25Q = 600
300 - 10Q + 25Q = 600
15Q = 300
Q = 20
D = 10
Check, it works.