The questioner was quite young and obviously must see both definitions used therefore it was a valid question.
The point is that he did not know the answer and for a minister who spends millions of our money it is appalling. He should at least know how to count money.
My second point is that Dumbleboy dismisses unexpected questions, questions are pre submitted and answers pre-prepared, just like PMQs. For this reason I no longer watch Question Time, far better and cheaper for BBC to list the answers for those interested and show a repeat of 'Del Boy' for entertainment.
Out of interest in my youth there were $3 to £1, a $ billion was worth ~£330,000,000 a £ billion was £1000,000,000,000. quite a difference, now that difference is minimal and falling.