Short answer : 90 × 4 = 360

Model

Key :
⬜ refers to fishballs sold in the afternoon
⬛ refers to fishballs left at the end of the day


90 ⬜⬛⬛


⅓ of the remainder can be thought of as :

Remainder = 3 units
Fishballs sold in the afternoon = 1 unit
Fishballs left = 2 units (you can also say it's ⅔ of the remainder)

Now the question says ½ of the fishballs are left at the end of the day.

This means 2 units are equal to ½ of all the fishballs.

Total fishballs = 2 units × 2 = 4 units

(Or simply, change ½ to 2/4 and notice the denominator 4, which represents all fishballs and the numerator 2 represents the leftover fishballs)

Since total fishballs = 4 units, then it is pretty obvious that the 90 sold in the morning has to be equal to 1 unit.

Great thanks!!!