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50.0 mL of 0.20 M Pb(NO3)2 contains .05*.02 = .01 moles
according to the equation, each mole of PbNO3 requires one mole of CaCl2.
so, since 1L of .25M CaCl2 contains .25 moles, you want .01/.25 = .04L = 40mL