哈斯娜（Hasnah）有 5 盒相同的橡皮擦。起初，每个盒子里的橡皮擦数量相同。她从每个盒子中拿走了 21 个橡皮擦。之后，5 个盒子中剩余的橡皮擦总数等于起初 2 个盒子里的橡皮擦总数。
起初每个盒子里的橡皮擦数量是多少？ 答案: ___

1. Set up the relationship between the total erasers taken and the boxes

Let $1$ unit be the number of erasers in $1$ box at first.
Total erasers at first = $5$ units.
Total erasers taken out = $5 \times 21 = 105$.

2. Equate the remaining erasers to the given condition

Erasers left = $5$ units $- 105$.
We are given that the erasers left equal the total in $2$ boxes at first, which is $2$ units.
So, $5$ units $- 105 = 2$ units.

3. Solve for 1 unit

$3$ units $= 105$
$1$ unit $= 105 \div 3 = 35$