I see the phrases “1/3 of the pens” and “3/4 of the rest”.

Since the denominators contain 3 and 4, I let the total number of pens be 3 x 4 = 12 units for easy computation.

Total number of pens is 12 units.

1/3 of the pens are red pens, so 4 units are red pens.

3/4 of the remaining 8 units of pens are blue pens, so 6 units are blue pens. This leaves 2 units for the green pens.

Since 6 units out of the 12 units are blue pens, 1/2 of the pens are blue pens.

12 units = 96 pens
1 unit = 96/12 = 8 pens
2 units = 8 x 2 = 16 pens

so there are 16 green pens.

Thanks a lot!