Remember

and
if

where x=x then m=n
so

so
m+n=-6
so test the options
first one works
2nd ones doesn't
3rd one doesn't
4th one doesnt'
answer is first option, m=3 and n=-9
Let A be the amount of 100% acid.
100A+20*(6)=40*(A+6)
100A+120=40A+240
60A=120
A=2 gallons
Let the original amount be x so ,
according to question,
3x/5 + 1x/5 + 1200 = x
or, 1200 = x - 4x/5
or, 1200 = 1x/5
or, x = 6000
volume = length x width x height
all 3 are the same = 2.5
so volume = 2.5 * 2.5 * 2.5 = 15.625 cubic units