Question 1001507
Call the number of dimes and nickels, d and n.
Thus the number equation is
d + n = 30
The value equation is
10d + 5n = 210  (we do it in cents)
Though we can divide this second equation by five and combine, let's multiply the first equation by five and subtract it from the second...
10d + 5n = 210
-(5d + 5n = 150)
and we get
5d = 60
d = 12 dimes
n = 30-12 = 18 nickels