Answer :

Millie has a box of 100 cubes.
Millie also has a bag of 70 cubes.
Total number of cubes Millie has = 100 + 70
                                                    = 170
Millie wants to make trains of 10 cubes each.
So the number of tains Millie can make = 170/10
                                                               = 17
So Millie can make 17 trains in total with the cubes she has.