6, 10, 12, 20. If you randomly draw balls from the pocket, at least how many balls must be drawn to ensure that at least 7 of them have the same number?
【Analysis】According to the worst-case principle, the number of balls numbered 1 and 2 is less than 7, so they should all be taken. Then, take 6 balls each of numbers 3, 4, and 5, and take one more. At this point, there must be a number of balls that have 7. Therefore, the total number of balls to be taken is $2+6+3 \times 6+1=27$.