At the beginning of the month, Frank had 21 boxes of pens with 13 pens in each box. This means that the total number pens that he Frank had at the beginning of the month is 13 × 21 = 273 pens.
At the end of the month, there were 45 pens left. The number of pens that he gave out will be the number of pens at the beginning of the month - the number of pens left at the end of the month. It becomes
