Afarmer planted 4 acres of soybeans for every 9 acres of corn. he planted a total of 400 acres. how many total acres of soybean did he plant? (round answer to the nearest whole acre.)

Lets have 4x acres of soybeans , then it is going to be 9x acres of corn
(4x/9x=4/9)
Altogether we have (4x+9x) acres 
Altogether he planted 400  acres
So, we can write
4x+9x=400
13x=400
x=400/13

4x acres of soybeans  
4*400/13≈123 acres of soybeans

This does not make sense

I'm pretty sure it's b). i try drawing the functions out and b is the one that stands out or isnt a "line"