Let r = small tier and s be medium tier.
Then 3r + s = 100 ------------(1)
And 3r + 2s = 140 ------------(2)
Subtracting (2) from (1) we have
s = 40.
Substituting value for r into (1) we have
3r + 40 = 100.
3r = 100 - 40 = 60
3r = 60
r = 20.
If U = 3r + s = 100 = 3r + s - 100
Then du/dr = 3 and du/ds = 1
Also, du/dr = 3 and du/ds = 2