Set x equal to the amount of $6 coffee
Set y equal to the amount of $3 coffee
Since the amount of total coffee is 180, x+y = 180
Since the total PRICE of the coffee is 180*4 (720), 6x+3y = 720
Use the first equation, x+y = 180, and set it equal to x or y. So x = 180-y or y = 180-x
Plug the equation in respect to x or y into the second equation, 6x+3y = 720 like this: 6(180-y) +3y = 720
6x+3(180-x) = 720
Solve for x or y depending on which equation you plugged in.
Example: 6x + 3(180-x) = 720
x = 60.
Plug x into the original equation, x + y = 180
y = 120.
Therefore: there is 60 coffees that are $6 and 120 coffees that are $3.