Question 876693
Learn to understand this kind of mixture problem in this lesson:
<a href="http://www.algebra.com/tutors/Two-Part-Mixture-with-one-material-quantity-unknown.lesson?content_action=show_dev">Mixture: one material quantity unknown </a>


Let v = how many gallons of 50% to add.
{{{highlight((50v+20*90)/(v+90)=40)}}}
Solve for v.