Question

A farmer grows corn, tomatoes, and sunflowers on a 320-acre farm. This year, the farmer wants to plant twice as many acres of tomatoes as acres of sunflowers. The farmer also wants to plant 40 more acres of corn than tomatoes. How many acres of each crop should the farmer plant?

Answer 1

tomatoes is 112 acres 
corn is 152 acres 
sunflower is 52

Answer 2

Set it up with variables; tomatoes=t, sunflowers=s, corn=c

320=t+s+c

This year;
t=2s   
c=t+40

so, replace 2s for t; c=2s+40 

now we can put those factors in for 320=t+c+s

320= 2s+2s+40+s
(SIMPLIFY)---> 320=5s+40----> 5s+280----> s=56 

now incorporate the s value into the other equations. 

c=2s+40----> c=56(2)=40 ----> c=152 
t=2s -----> t=2(56) ----> t=112

TOMATOES= 112 acres
CORN= 152 acres
SUNFLOWERS= 56 acres

To check your work: 320=t+s+c ----> 320=112+156+56