Question 366383
Let {{{x}}} = gallons of 4% solution to be drained
given:
Bleach in 4% solution:
{{{.04*120 = 4.8}}} gal
Bleach in removed portion:
{{{.04x}}}
Gallons of bleach added:
{{{x}}}
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In words:
(initial gallons of bleach - bleach removed + bleach added)/(total gallons) = 6% 
{{{(4.8 - .04x + x)/120 = .06}}}
{{{4.8 - .04x + x = 7.2}}}
{{{.96x = 2.4}}}
{{{x = 2.5}}}
2.5 gallons of 4% solution must be drained
and replaced with pure bleach