Question 717000
Assign Variables:
H = high concentration, 80%
L = low concentration, 30%
v = amount of low concentration material being used and available, 70 gallons
T = Target concentration wanted, 70%
g = how many gallons of the 80% material to add, UNKNOWN


Form equation:
{{{(vL+gH)/(v+g)=T}}}, solve for g.  Substitute given values.