B for Brats and H for Hamburgers
3 total for each of 30 guests
B + H = 3x30 = 90
Then $1.25B + $0.75H <= to $120
Solving for B=90-H
1.25B + 0.75(90-B) <=120 yields cost of B is $52.50 so B = 42.
90-42 = 48 H
So you can buy 42 Brats and 48 Hamburgers
Lets assume, Sally has X pound of potatoes in her pantry.
So, 4/3 of the potatoes Sally has in her pantry = 
Now, Sally needs 2/3 pound of potatoes and this 2/3 pound of potatoes is 1/3 pound more than that 
So the representing equation will be:
(subtracting
from both sides)

(multiplying both sides by 3)
So, Sally has
pound of potatoes in her pantry.