The number of B that will be needed to convert 5 moles of A into as many moles of C as possible is 20 moles
Explanation
3A + 12 B → 4 C
The moles of B is calculated using the mole ratio of A to B from equation above
that is the moles ratio of A:B = 3 : 12
Therefore the moles of B = 5 x12/3 = 20 moles