initial
f+u+d = 18
f/15 + u/6 + d/24 = 1.5

f+2u = 18
f/15 + 5u/24 = 1.5
f = 15(1.5-5u/24)
f = 22.5-75u/24

22.5-75u/24 + 48u/24 = 18
4.5=9u/8
u=4
d=4
f=10