so, if he wants to make [tex] \frac{7}{9} [/tex] more concrete, he should use [tex] \frac{7}{9} [/tex] more water!
so he should use 1 of the planed water plus [tex] \frac{7}{9} [/tex] of the planned water, that is [tex] \frac{9+7}{9} [/tex] or [tex] \frac{16}{9} [/tex] of what the formula says.
so we multiply this by what the formula says:
[tex] \frac{16}{9}*\frac{3}{8}=\frac{2}{9}*\frac{3}{1}=\frac{2}{3}*\frac{1}{1} [/tex]
so he has to use 2/3 gal of water
