How much sodium phosphate buffer (1.5M, pH 7.2) is required to make 125 ml of a 50 Mm solution? How much water?

mL x M = mL x M

mL x 1.5M = 125 mL x 0.050 M
Solve for mL = 125 x 0.050/1.5 = 0.625 mL of the stock.
So you will need 125 mL - 0.625 mL = x mL water.