$3, 6, 15,$ and $18$ are multiples of $3$. Thus, the probability is $\frac 2{5}$. 
There are $$10$$ cards in a box, which numbered $$1$$, $$3$$, $$4$$, $$6$$, $$7$$, $$11$$, $$15$$, $$16$$, $$18$$, and $$20$$, respectively. Cathy picks a card from the box randomly. What is the probability the number on the card that Cathy picks is a multiple of $3$? 
(a) $\frac 1{10}$ 
(b) $\frac 3{10}$ 
(c) $\frac 2{5}$ 
(d) $\frac 1{2}$ 
(e) $\frac 3{5}$ 