Question 1073353
Look at the outcomes that are multiples of 3 : {3,6,9,12,15,18}. There are 6 of them.
Look at the outcomes that are multiples of 5 : {5,10,15,20}. There are 4 of those.
Look at the outcomes that are both : {15}.
So out of 20 possible outcomes, 
{{{P=(6+4-1)/20=9/20}}}