Let the number of hamburgers = h
Let the number of cheeseburgers = c
Now we translate the given information into equations.
"combined total of 770 [ham]burgers and cheeseburgers"
h + c = 770
"there were 70 more cheeseburger sold than hamburgers"
c = h + 70
Our system of equations is
h + c = 770
c = h + 70
The second equation is already solved for c, so we replace c of the first equation with h + 70.
h + h + 70 = 770
2h + 70 = 770
2h = 700
h = 350
Answer: 350 hamburgers were sold on Monday.
