For lunch Jack had a big mac and two small fries containing 1000 calories while randy had three big macs and two small fries containing 2080 calories how many calories are in big mac?

The number of calories in a single Big Mac is ##540##.
We consider the Big Mac as ##x## and the two small fries as ##y##. From the data:
##x+y=1000##
##3x+y=2080##
Subtracting the first equation from the second equation, we get:
##(3x+y)-(x+y)=2080-1000##
Open the brackets and simplify. The product of a negative and a positive is a negative.
##3x+y-x-y=2080-1000##
##2x=1080##
Divide both sides by ##2##.
##x=540##