I. CaO + HCl  CaCl2+ H2O

If you compare the two sides you can see that to balance the Ca's you need the same number of CaO as CACl2. And if you compare the O's you see that you have the same number of CaO as H20. Finally look at the H's - not that you need twice as many HCl as H20, and similarly to balance Cl's you need twice as many HCL as CaCl2. So - what does this tell you about what number to put in front of the HCl?