$Q _ { s } = 40 + 6 P - 8 P _ { I } + 10 F$

$Q _ { s } = 40 + 6 P - 8 P _ { I } + 10 F$ where $Q _ { s }$ is the quantity supplied of the good, P is the price of the good, $P _ { I }$ is the price of an input, and F is the number of firms producing the good.
-Again suppose $P _ { I }$ = $40 and F = 50, what is the lowest price that will induce firms to supply 400 units of output?

A) $15
B) $20
C) $25
D) $30
E) $35