Xiaoling: 120*( 1/4)=30 trees.
Xiaoming: 120*( 1/5)=24 trees
therefore
Xiaohong: 120=40-30-24=26 trees.
Explanation: "The number of trees planted in a small square is half of the total number of trees planted by other three people"-the number of trees planted in a small square can be regarded as 1, and the total number of other three people can be regarded as two, so the total number is ***3, and the small square accounts for 1/3 of the total. So the number of trees planted in a small square is 120*( 1/3)=40, and so on.