The total amount of the two types being mixed must add to 50 gallons.
So if we use x gallons of 18% fertilizer
We then add to that (50-x) gallons of 30% fertilizer
Look at the concentrations of actual fertilizer.
We have 0.18x gallons of actual fertilizer, added to 0.3(50-x) gallons of
actual fertilizer.. summing to 50 gallons of 27% fertilizer which is 0.27(50)
Your equation is:
0.18x + 0.3(50-x) = 0.27(50)
Multiply through by 100 to clear the decimals
18x + 30(50-x) = 27(50)
18x + 1500 - 30x = 1350
-12x + 1500 = 1350
If you finish solving for x from this point that is the number of gallons
of 18% fertilizer solution.
Then use 50-x to find the number of gallons of 30% fertilizer solution.