First we need to find the probability of selecting the 1 blue marble out of the 20 marbles total, which is 1/20.
Next, we need to find the probability of tossing the fair coin so it lands tails up, which is 1/2.
To find the probability of both events happening, we need to multiply the probabilities of the two individual events:
1/20 * 1/2 = 1/40
So the answer is 1/40, or 2.5%
Sorry about the poor formatting :( I'm on my phone at the moment, which makes it difficult.
Hope I helped, and let me know if you have any questions :D