What is the expected number of marked fishes after 7 times? Given 10 fishes, each time one fish is chosen randomly, marked and returned to the pool. If a fish is already marked, it constitutes as a turn, and returned the the pool as well.
What is the expected number of marked fishes after 7 times?
Is it:
For each fish,
P(marked by 1st turn) OR  P(marked by 2nd turn)... OR P(marked by 7th turn) = $\frac{1}{10} * 7$
Hence, for ten fishes, the expected number of marked fishes after 7 times (by linearity): $$\frac{7}{10}*10=7.$$

This is a similar question from Brilliant.org where it asked about:
What is the expected number of unmarked fishes after 7 times?
The thought process would be for each time to not be marked: $\frac{9}{10}$.
So, P(unmarked in 1st) AND P(unmarked in 2nd) ... AND P(unmarked in 7th) = $\frac{9}{10}^7.$ Then it would be
$$\frac{9}{10}^7*10$$
Part of what I would like to clarify is whether my way of thinking is correct between AND and OR; not AND: multiply and OR: sum, but rather, did I construct the solution correctly for the marked version? My initial immediate thought for the marked fishes was to use $0.1^7$ instead of $\frac{1}{10} * 7$.
Sorry if what I am asking is unclear.

To answer your question about which thought process is correct: The AND one is, the other (first presented) is wrong. That can be seen by looking critically at your result that after 7 marking rounds, the expected number of marked fish is supposed to be 7, when that is actually the maximum number you can hope for.
Generally, if you have 2 events $A$ and $B$ and know $P(A)$ and $P(B)$, you need different kinds of assumptions to be able to calculate $P(A \cup B)$ and $P(A \cap B)$ just from $P(A)$ and $P(B)$.
In order for $P(A \cup B) = P(A) + P(B)$ to hold, you need to know that events A and B are disjoint, meaning they can't both happen. That's why the OR approach is incorrect, you can mark the same fish in the first and 3rd turn, for example, so the events whose probabilities you added were not disjoint.
In order for $P(A \cap B) = P(A)P(B)$ to hold, the events must be independent. That's why the AND approch works: Knowing if a given is fish being (un)marked on turn 1 does not tell you anything if it is being (un)marked on turn 2 or any other turn.
