Hasnah had 5 boxes of identical erasers. At first, each of the boxes contained the same number of erasers. She took 21 erasers from each box. After that, the total number of erasers left in the 5 boxes was equal to the total number of erasers in 2 of the boxes at first.
What was the number of erasers in each box at first? Ans: ___

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 $= 105 \div 3 = 35$