x represents the number of tacos and each taco costs $4 so the total earnings for selling tacos is 4x
y represents the number of burritos and each one costs $8, so the total earnings for selling burritos is represented as 8y
If you add the earnings of tacos and burritos you get the total earnings of the day 4x+8y
He must sell a minimum of $780, this means that he must earn $780 or more
Symbolically:
The maximum number of tacos he can sell is 20, this is, 20 tacos or less, you can express this as
He only has supplies to make a maximum of 130 tacos or burritos, so that
Plot the three inequalities
One possible solution is where the three shaded areas met, for example he can make 20 tacos and 100 burritos