Question 1125126
.
<pre>
Let x be the amount of the 90% disinfectant (in gallons).


Your equation is 


    <U>pure disinfectant</U> + <U>pure disinfectant</U> = <U>total pure disinfectant</U>,     or


    0.7*100           + 0.9x              = 0.75*(100+x),        


    0.7*100           + 0.9x              = 0.75*100 + 0.75x


    0.9x - 0.75x = 0.75*100 - 0.7*100


    x = {{{(0.75*100 - 0.7*100)/(0.9 - 0.75)}}} = 33.333 gallons.


<U>Answer</U>.  33.333 gallons of the 90% disinfectant.


<U>Check</U>.   0.7*100 + 0.9*33.333 = 100 gallons of the pure disinfectant;

         0.75*(100+33.333) = 100 gallons of the pure disinfectant.   ! Correct !
</pre>

Solved.