The good seed flower shop sells its own brand of fertilizer. it mixes one brand of fertilizer that contains 7% nitrogen with another than contains 8% nitrogen. the resulting mixture contains 7.7% nitrogen. how many pounds of the 7% fertilizer must be used to make 400 pounds of the 7.7% mixture?

X - lb 7% fertilizer
y - lb 8% fertilizer
1) x+y = 400
2) 0.07x - lb nitrogen in x lb  7% fertilizer
0.08y - lb nitrogen in y lb  8% fertilizer
0.077*400=30.8 lb nitrogen in 400 lb  7.7% fertilizer
0.07x + 0.08y = 30.8

System of equation:
x+y = 400 /*(-0.08) > -0.08x-0.08y =-32
0.07x + 0.08y = 30.8

-0.08x-0.08y =-32
0.07x + 0.08y = 30.8    Add 2 equations.

-0.01x   = -1.2
x=120 lb 7% fertilizer must be used