The vapour pressure of two liquids P and Q are 80 and 60 torr respectively. The total vapour pressure of solution obtained by mixing 3 moles of P and 2 moles of Q would be:

Mole fraction of P = 3/(3+2) = 3/5
Mole fraction of Q = 2/(3+2) = 2/5
Hence total vapour pressure = (Mole fraction of P × Vapour pressure of P) + (Mole fraction of Q × Vapour pressure of Q)
= (3/5 × 80 + 2/5 × 60)
= 48 + 24
= 72 torr
Hence, the correct option is D