Let the children understand that the locomotive is the sum of two numbers. Then in this case, the sum of the upper and lower numbers should be the number on the locomotive.

The first locomotive is 5 1/4, 2/3, 2/3, 1/4. The sum of the upper and lower numbers is 5.

The second locomotive is 6,5/1,4/2,3/3,2/4, 1/5. The sum of the upper and lower numbers is 6.

The third locomotive is 7,6/1,5/2,4/3,3/4,5/2,6/1. The sum of the upper and lower numbers is 77.