equations

One sort of fertilizer contains 10% nitrogen , and the other fertilizer contains 19% nitrogen. How much do you need to take of both sorts of fertilizer so you can get 450 kilos of mixture which contains 14 % of nitrogen . 

https://picload.org/image/rpapppda/25.jpg

Let the number of pounds needed from the 10% fertilizer = T
Then number of pounds needed from the 19% fertilizer =450 - T
0.1T + 0.19(450 - T) = 0.14(450), solve for T
T = 250 pounds of 10% fertilizer.
450 - 250 = 200 pounds of 19% fertilizer.