How to solve these Chemistry question? Involving moles?

(8)  The HCl solution is twice as concentrated as the Ca(OH)2 solution, but it takes two moles of HCl to react with each mole of Ca(OH)2.  Therefore the two volumes can be exactly equal -- the answer is 25.0 cm^3 (answer D).

(9)  You want to dilute the solution by a factor of (0.2/0.05) = 4, so you must add three times the original volume, that is, 75.0 cm^3 (answer B).