an orange has 20 fewer calories than a banana. If 7 bananas have the same number of calories as 9 oranges, how many calories are in a banana?

Answer 1

Let X be the number of calories in a banana, and Y be the number in an orange.
7X = 9Y
7X - 9Y = 0
X - Y = 20
9X - 9Y = 180
2X = 180
X = 90
Y = X -20 = 70