Our son counted to one hundred for his homework, by way of twenty-ten

The task was to count aloud to one hundred and have a parent sign the diary. He went smoothly to twenty-nine, paused, said 'twenty-ten' with complete confidence and carried on. Twenty-eleven, twenty-twelve, all the way to twenty-nineteen before my partner, who is a teacher, stepped in. When corrected, his position was that his way is more logical. I am an architect and I find it difficult to disagree. The diary has been signed, with a note.