If 6 is a multiple, we need 2 and 3 as a factor of the number. If we factor the numbers 11 through 20 we have:

11 = 11
12 = 2*2*3
13 = 13
14 = 7*2
15 = 5*3
16 = 2*2*2
17 = 17
18 = 3*2*3
19 = 19
20 = 5*2*2

Combos that have a 2 and a 3 as factors (first + second card):

12 + anything - 19 of these possibilities (we count 12 + 12 only once)
14 + 15 - 2 of these possibilities
15 + 16 - 2 of these possibilities
15 + 20 - 2 of these possibilities
18 + anything (except 12, since that is counted in our first option) - 17 of these possibilities

Total possibilities that will be a multiple of 6: 42
Total possibilites: 10*10=100

Probability = 42/100 = 0.42

Answer: D

James, I did it basically the same way but still took me 2:26 to get the answer. What would be the quickest way to figure this problem out?

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