Think of pens at first as 4 units

After selling ¼ of her pens (which is 1 unit), 3 units of pens are left.

After selling 108 pencils, the number of pens left = number of pencils left = 3 units

Pens and pencils at the end
= 3 units + 3 units
= 6 units

Now 374 - 108 = 266

So,

6 units = Total number at first - 108 - 1 unit
6 units = 374 - 108 - 1 unit
6 units = 266 - 1 unit

Now what if she didn't sell any pens?

There will be 7 units left instead.
(4 units are pens and 3 units are the pencils)

266 = 7 units

1 unit = 266 ÷ 7 = 38

Pens at first = 4 units

= 4 × 38
= 152

OR

Pens at first
= Total - pencils
= 374 - 3 units - 108
= 374 - 3 × 38 - 108
= 374 - 114 - 108
= 260 - 108
= 152

Model would look like this :


At first,

Pens→⬛⬛⬛⬛
Pencils⬛⬛⬛108

Total = 374

After,

Pens→⬛⬛⬛
Pencils⬛⬛⬛


We can see that :

374 - 108 = 7 units
266 = 7 units

Thank you:-)