You are indeed overthinking. The probability of drawing the marble u moved into jar 2 would be 1/10. The probability of picking a blue marble to move would have been 6/11. So the probability of drawing the marble u moved and it being blue is (1/10)(6/11) = 6/110. The probability that a blue marble that was originally in jar 2 was picked up is, 5/10. So 6/110 + 5/10 is the probability of picking a blue marble from jar 2