rLPG / rSO3 = √(80/ MLPG) = 1.25 
Therefore, 80/M = (1.25)2 
Therefore, MLPG = 51.2 
M = (M1(butane ) X1 + M2(propane)X2) / (X1 + X2) 
51.2 = 58X1 + 44(1-X1) / 1 
X1 ≈ 0.50