My estimate is 24.
I think that 10 is too small, 40 is too big, so 36 is my estimate.
I think this pencil case is going to be a 'Tardis' - bigger on the inside than the outside! An estimate that is too small is 30 and an estimate that is too big is 60. How did I do?
I think 20 is too small, 40 is too big and my estimate is 30