Mercury in U-tube problem

Mercury is poured into a U-tube. Left arm has a A1= 10cm^2, and the right arm has a A2=5cm^2. 100g of water is poured into the right arm. The length of the water column is 20cm. Given the density of mercury is 13.6g/cm^3, what distance does the mercury rise in the left arm?

So the answer is 0.490cm, how do you get this answer?

If U tube is symmetric then rise by h in left arm would have meant fall by h in right arm so that the difference in mercury column balancing water column would have been 2h. but in this case the area of cross-section of left is twice that of right hence a rise by h in left will mean fall of 2h in right so that the level difference of mercury will be 3h. So we have
(3h)*13.6 = 20*1 or h = 20/(3*13.6) = 0.4901 cm or 0.490 cm