Question 741108
Another hint is these two question:
How much bleach?  How much mixture?  The ratio is the concentration of bleach.


How much bleach:  2*5%+3*20%


How much mixture:  2 gallons + 3 gallons = 5 gallons


Percent bleach in the mixture:  {{{(2*5+3*20)/(2+3)=PercentBleachMixture}}}