ok im a lil stuck on this problem...

M gallons of a P% salt solution must be mixed up with G gallons of a q% salt solution to produce an r% solution. how do you find r?

ok so the first part is Mp/100 and the second part would be Gq/100. the trick is that both parts are added to get the r% which is what i want.

so i have:

after adding i have:

but this is where i get stuck im not sure where to go from here..any thoughts? my work is not looking like what the book has an answer..

thanks in advance.