5:3=20:12
4:7=12:21
则苹果、梨和橘子的筐数之比为20:12:21
则:2120÷(20+12+21)
=2120÷53
=40筐
则苹果有:40×20=800筐
梨有:40×12=480筐
橘子有:40×21=840筐