If you were to solve this problem, you would need more information, like the number of calories in a banana. But, you can still use equations to solve this. Assume a banana has Y calories, which means an orange has Y-20 or x + 20 calories. The 2nd equation would be 7y = 9x. Solve equation two to get Y = 9x/7 and then use this in equation 1 to get X = 70 and Y = 90.