Question 134535
In words:
(gallons of isopropyl) / (gallons of mixture) = 16%
Let {{{x}}} = amount of 10% mixture to be added
{{{60}}} gallons = amount of 20% mixture added
The total gallons of mixture  = {{{x + 60}}}
{{{(.1x + .2*60) / (x + 60) = .16}}}
{{{(.1x + 12) / (x + 60) = .16}}}
multiply each sides by {{{x + 60}}}
{{{.1x + 12 = .16(x + 60)}}}
{{{.1x + 12 = .16x + 9.6}}}
{{{.06x = 2.4}}}
{{{x = 40}}}
He added 40 gallons of the isopropyl mixture
check:
{{{(.1x + .2*60) / (x + 60) = .16}}}
{{{(.1*40 + .2*60) / (40 + 60) = .16}}}
{{{(4 + 12) / 100 = .16}}}
{{{16/100 = .16}}}
{{{.16 = .16}}}
OK