You’ll need to use the conditional probability formula. In the formula, outcome A = “dart pierces both cards”, and outcome B = “dart pierces at least one card”. The probabilities are directly related to the surface areas of the four possible regions (no card, card 1, card2, both cards) the dart can stick in.

(It is worth noting that the whole problem isalreadyone of conditional probability because it implicitly assumes that the dart sticks somewhere in the 30 × 30 cm board.)