Answer:
2.5 kg
Step-by-step explanation:
We can solve the problem by using Newton's second law:
![F=ma](https://img.qammunity.org/2020/formulas/physics/middle-school/hoqv0uuwk5hamoxytydy5e8slsjemaiqzz.png)
where
F is the net force acting on the train
m is the mass of the train
a is the acceleration
For the toy train in the problem,
F = 3.0 N
a = 1.2 m/s^2
So we can solve the formula for m, to find the mass of the train:
![m=(F)/(a)=(3.0 N)/(1.2 m/s^2)=2.5 kg](https://img.qammunity.org/2020/formulas/physics/middle-school/fow3al3a5mknl5fo4qbf3h80matb7wkar9.png)