Question 859700
Main interest is about percent paint, starting with 10 gallons of 90% paint.

Let y = volume of water to use, to add to the 10 gallons of paint, to obtain 80% paint.


Only one needed equation.
{{{highlight((10*90)/(10+y)=80)}}}
Solve for y.


Percents were used, although the equation could have been done completely as decimal fractions.  


It that equation as shown, enough for your son, or is more explanation or steps needed?