Quantity eq.: d = n+9
quantity eq.: q = d+21
value equation: 5n + 10d + 25q = 1000

rearrange:
d = n+9
q = d+21 = n+9 + 21 = n+30
substitute to get:
5n + 10(n+9) + 25(n+30) = 1000
40n + 840 = 1000
does this look correct?
40n = 160
n = 4 (# of nickels)
d = 4+9 = 13 (# of dimes)
q = 4 + 30 = 34 (# of quarters)