Martin flips a standard coin three times, what is the probability it lands on tails all three times?

thirty-three percent

At the seventh meeting there would be 20 members. If you think about the question carefully there were 12 members at the first meeting, then they skipped 6 meetings, with each meeting being skipped or not written down they added 1 member, so you would add 2 members for the next 2 meetings. making 20 members. 
Hope this helps!