6% of weight of non volatile solute in isopentane(M=72) at 300K has V.P of 1426mm of hg.20g of isopentane is now added to the solution and the resulting solution has  vapour pressure(V.P) of 1445.calculate V.P of isopentane

P = Pº x mole fraction 
6% by mass means 6g of 100g solution = solute 
94 g isopentane / 72 g/mole = 1.3 moles 

adding 20g isopentane 
6g solute in 120g solution where 114g = isopentane 
114 g / 72 g/mole = 1.58 moles 

1426 = Pº x 1.3 / (1.3 + y) 
1445 = Pº x 1.58 / (1.58 + y) 
set equations = to each other since Pº is constant and y (moles solute) is constant 
1426 x (1.3 + y) / 1.3 = 1445 x (1.58 + y) / 1.58 
y = 0.104 moles solute 

P = Pº x mole fraction isopentane 
1426 = Pº x (1.3 / (1.3 + 0.104)) 
Pº = 1540.1 mmHg