A chemical experiment requires that a solute be diluted with 4 parts (by mass) water for every 1 part (by mass) solute. If the desired mass for the solution is 90 grams, how many grams of solute should be used?

The ratio of solute to solution is 1:5.
Write a proportion and solve.
Let the grams of solute should be used be "x", plug in the ratio above we have:
\( \frac{\mathrm{1} }{\mathrm{5} } = \frac{\mathrm{x} }{\mathrm{90} }\)
\(x = \frac{\mathrm{1} }{\mathrm{90} } = \frac{\mathrm{x} }{\mathrm{5} }\)
\(x = 18\)