Why do this problem?
will help pupils to apply and improve their knowledge of number bonds and to develop systematic ways of working.
The link between the Cuisenaire rods and the number they represent could be made more explicit by showing the rods on $1$ cm squared paper. Ideally, this could be done in front of the whole class using OHT rods and a grid.
Which is the longest rod? Could you try with that one first?
How about the next longest?
How many white rods are there in Laura's train?
How many different ways can you find to make a train the same length as Laura's, with three rods which are all different colours?
How can you record what you have done?
Would it help to use squared paper?
Those who find this easy could find all the three-colour trains in all lengths from $6$ to $20$ or try Cuisenaire Counting
Suggest drawing round the Cuisenaire rods on $1$ cm squared paper and then colouring them.
Same Length Trains
is a simpler problem that some pupils might like to attempt before trying this problem.