Equation = Ca(OH)2 + 2HCl -> CaCl2 + 2H20
36.0 ml*(0.05M/1ml) = 1.8 mmol HCl
since it requires 2moles of HCl to react with 1 mole of Ca(OH)2
1.8/2= 0.9mmol is need
0.9mmol/50ml of Ca(OH)2 = 0.018mol
Here is your answer: 0.018mol
If you still dont understand you can go on Chemistry - Google ¹Ï®Ñ to page 384 of the book.