Number of possible outcomes: 20*12 = 240

Assumptions: Unbiased dice, independent dice. The assumptions guarantee that each of the outcomes is equally likely.

Thus each outcome has a probability of: 1/240

Sums less than 10: 2, 3, 4, 5, 6, 7, 8, 9

Outcomes that give a sum of 2: 1-1

Outcomes that give a sum of 3: 1-2, 2-1

Outcomes that give a sum of 4: 1-3, 2-2, 3-1

Outcomes that give a sum of 5: 1-4, 2-3, 3-2, 4-1

Outcomes that give a sum of 6: 1-5, 2-4, 3-3, 4-2, 5-1

Outcomes that give a sum of 7: 1-6, 2-5, 3-4, 4-3, 5-2, 6-1

Outcomes that give a sum of 8: 1-7, 2-6, 3-5, 4-4, 5-3, 6-2, 7-1

Outcomes that give a sum of 9: 1-8, 2-7, 3-6, 4-5, 5-4, 6-3, 7-2, 8-1

Total number of outcomes in our EVENT: 1+2+3+4+5+6+7+8 = 36

The probability of this event is: 36/240 = 3/20 = 0.15 or 15%