"If you give four classes, there are still three left; If you give five classes, there are still four left. " This means that the number of oranges is divided by four to three, and divided by five to four. It can also be said that the number of oranges divided by 4 is 1 and divided by 5 is 1.

Find the common multiple of 4 and 5 minus 1, and that's it.

Common multiples of 4 and 5 are 20, 40, 60, 80, 100 and 120. .....

Choose 100 according to the meaning of the question.

100- 1=99 (orange)