All the above solutions are correct, but I haven't thought for a long time and come up with a suitable way to explain to this child without X. In the final analysis, this kind of primary school question is a very Austrian logical explanation question. Of course, adults can give him a result with knives, guns and sticks, but children can only use these tools with their bare hands, so let me think again. ...

Come back, try to explain it and see if the child can understand it logically. If he understands, it's OK.

Arithmetic calculus:

40+24=64

64/2=32 months

40-32=8 Tingting

Logical explanation:

Two fists can be used for analogy. The left hand is the age of mother+Tingting (40) and the right hand is the age of mother-Tingting (24). So if these two fists are combined, the age of left hand+Tingting and the age of right hand-Tingting just offset, then the rest is twice the age of mother, divided by 2 to get the age of mother, and then subtracted from the age of mother and Tingting and the age of mother to get Tingting's age. That explains this.