lowering of vapour pressure

I used it but i didn't get the answer.

I get the answer 27 but it is given 36

(p⁰-p_s)/p⁰=n₂/n₁ is derived from (p⁰-p_s)/p⁰=n₂/(n₁+n₂) for n₂<<n₁
Here if you calculate from the former you get n₂=1/3 which is not much less than n₁.
So you have to use (p⁰-p_s)/p⁰=n₂/(n₁+n₂)
Now if calculate using this you will get n₂=1/2
M=18

sorry i have given the wrong answer, it is 18
Can anyone tell me when to use (p⁰-p_s)/p⁰=n₂/(n₁+n₂) and not (p⁰-p_s)/p⁰=n₂/n₁

are arman, apne arman o ko sambhalo, see the mass of water,1 mole to hai bechara...yani ek mole...aur solute itna kam to hoga nahi k 0.00001 ho uska mole fraction...itna to Q dekh k pata chalta hai....dat means...