Long multiplication: Recording zero after a placeholder
When multiplying by the tens digit, a placeholder must always be used to maintain place value. This means that in the second row of the multiplication, if multiplying the units by the tens digit gives an answer ending in zero, there will be two zeroes at the end of the multiplication.
In this example, the pupil has either forgotten to write in the placeholder or failed to record the additional zero resulting from 2 x 5 = 10. They need prompting to see how the value of the digits has been lost (20 x 5 = 100).
The answer in the second (tens) row of the multiplication will always be bigger than the answer in the first (units) row. Understanding this can enable such mistakes to be quickly spotted.