You need a 75% alcohol solution. on hand, you have a 1125 ml of a 90% alcohol mixture. how much pure water will you need to add to obtain the desired solution?

so.. whatever "x" is, we know 1125 + x = y
and whatever the alcohol concentration yield may be, we know

(1125)(0.9) = 0.75y

(1125)(0.9) = 0.75y   <--- solve for "y", that's how much the final amount of milliliters will be

how much pure water was added? subtract 1125 from that, or y - 1125