With "dice" problems, there are no neat formulas.
Usually, we must list the outcomes (or visualize them).
I assume we are to find and .The number showing on the upper faces when two fair six-sided dice are tossed, is observed.
Consider events A and B
A: the total score is at most 5
B: at least one die achieves a score of 3 or 4
(1) There are: . possible outcomes.
There are 10 outcomes with a sum of 5 or less:
(2) I have a "back door" approach to the second part.
The probability that a die does not show a 3 or 4 is: .
The probability that neither die shows a 3 or 4 is: .
Therefore, the probability that at least one die shows a 3 or 4 is: