algebra
posted by katherine .
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?

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