80% of 90 may be found by multiply 90 by 0.8. Once you have that number, you can use it to set up a proportion with 2/5 (the simplified form of 40/100). Thus, you will write out 72/x = 2/5, since x is the number that you're solving for. Multiply numerators and denominators of opposite fractions to get the equation 72(5) = 2x. Finally, simplify 360 = 2x by dividing both sides by 2 to isolate the variable. You should have your answer by now.