Which is smaller, Se, or O?

Oxygen is much smaller then Selenium, that is because of how high the charge density is.

If you mean electron then O(8 electrons) is smaller than Se(34 electrons).

If you mean atomic mass then O(16) is still smaller than Se(78).

If   Valency then O and Se are the same(-2).

the answer is 1260

you spend 7 hours in school for 1 school day with 5 school days in 1 school week with 36 school weeks in a school year.

7*5*36= 1260 it's all about multiplying