In your question you wrote that those who chose pasta were 20 more than chose pasta so I'll assume you meant "than chose hamburgers"... That said, if you let salad = X then ha.burgers = 6x and pasta = 6x + 20
If we add them together we will get the total orders of 280, so our equation is
x + 6x + (6x + 20) = 280
13x + 20 = 280
13x = 260
x = 20
Remember x = salad so 20 people ordered salad
just for your info...
hamburgers would be 6x or 6 × 20 = 120 and
pasta would be 6x + 20 or 6×20 +20 = 140