We had lots of answers to this problem, however, many of you did not give a full solution. Please remember that we want to see HOW you arrive at the answer, not just the final result. Good explanations were sent from James and Chris from Little Aston Primary School, Rachel from Aldermaston C of E Primary School, and Ryan, James, Adam, Joseph and Richard from Moorfield Primary School. Here is what Rachel said:

If two circles are in a rectangle and the top, bottom and sides are touching the top, bottom and one side of the rectangle, you can then halve the diameter to get the radius of each circle (5cm÷2=2.5cm).
After that double the radius (2.5cm) for two circles and take that away from the length of rectangle (9cm), finding the distance between the centres of the two circles.
The answer is 4cm.

Pupils from Moorfield also drew the diagram to scale to check their answer, which is a good idea. They went on to say:

If you make the circles closer together or further apart, you can still find the gap between the two circles using the same method for e.g. the width was still 5cm but we changed the length to 7cm so we did this sum 7cm (Length) - 2.5cm (Radius) - 2.5cm (Radius) = 2cm (The gap between the two circles).
To cover the circles completely the length and the width would be 5cm.

Adam and Sam from Perton Middle School sent a very clear diagram to show the circles being moved apart:

Thank you to everyone.