Water makes about 3 4 of a person’s weight.
if a student weighs 100 lbs., how much of his weigh is water?

The student has 75 lbs of water from the 100lbs total weight (or 75% of the 100pounds)

There are several ways to find this out. The quickest way to find the answer is that you would want to multiply 3/4 by 100 to see what value is 3/4 of 100. If you put 3/4 into a decimal you would get 0.75 and then multiply by 100. 0.75*100=75 (move the decimal point to the right twice so that you get 75 because it is multiply by 100.)