a group friends ordered 3 burgers and 3 medium fries for a total of 20.25 .Another group of friends ordered 5 burgers and 8 medium fries for a total of 38.25.what is the cost of one burgers

a burger is $5.25, fries are $1.50

b = burger, f = fries
3b + 3f = 20.25
5b + 8f = 38.25

First, solve for b in terms of f
3b = 20.25 - 3f
b = 6.75 -f

Then substitute that for b in the other equation to solve and get a numerical value for f.

5(6.75 -f) + 8f = 38.25
33.75 -5f +8f = 38.25
33.75 + 3f = 38.25
3f = 4.5
f = 1.5

Now substitute that in to get a numerical value for b
b = 6.75 - 1.5
b = 5.25

Checking the math:
3b + 3f = 20.25
3(5.25) + 3(1.5) = 15.75 + 4.5 = 20.25