ANSWER

7-10 year olds

Keenan needs six cubes for his wagon. Then, he needs to grab 6 cubes.

Each cube is in the fifth new wagon of Mary.

a) Mary’s train has 5+5+5+5+5+5 = 10+10+10 = 30 wagons.

b) The number of complete wagons that she has is  4+4+4+4+4+4 = 24,

c) Each row of 5 wagons has  6+6+6+6+5 = 29 cubes. There are 6 rows. So,

in total there are   29+29+29+29+29+29 = 154 cubes.