Since the given equation is balanced, we know the Ca and the H2O are in a 1:2 ratio (to see the ratio, look at the coefficients in front of the molecules). Then, there are a few different ways to solve the problem. I am going to show you how to solve it using a ratio of moles of Ca/moles of H2O: x/1=4/2. First, you cross multiply to get 2x=4. Then, you divide both sides by 2 to get x=2 moles of Ca.