This activity appeals to many pupils much more than being presented with "sums" to do. It may make use of number bonds and facts that the pupils already know.

Use something fairly large to represent the $24$ carriages - even carriages from a toy train set would be great! You could create an IWB file that allowed you to create multiple copies of the carriage and to move them around the screen.

Encourage children to suggest some ways of making the trains to start with and display them using whatever you have chosen. If possible, keep these to be referred to later. Give children time to work in pairs on the challenge. You may want them to put each solution on a separate strip of paper, because then you could use these in the plenary to order the solutions in some way and this will help the group work out if they have missed any out.

How many carriages here?

Which train has most carriages?

How many carriages have you used?

Explore the results for $20, 21, 22$ and $23$ carriages, and compare them.

Encourage children to ask "I wonder what would happen if ...?".

Some of the youngest pupils may need help in counting accurately and not counting the same carriages twice.