Let x=number of cups of iced tea and y=number of cups of lemonade.
0.5x+0.8y=200 expresses the requirement in linear form.
This can be written 5x+8y=2000 and can be graphed as a straight line. To find the intercepts, put x=0 to find the y intercept and vice versa. When x=0, 8y=200, y=250 and when y=0, x=400.
The graph represents all possible solutions but we only need to look at part of it, the part that has x and y both positive. And we can rule out fractional values for x and y.
To find all solutions use values of y=0, 5, 10, 15, etc or values of x=0, 8, 16, 24, etc to find whole number solutions. Example: x=152, y=(2000-760)/8=1240/8=155. So the total number of cups is 152+155=307.