To find the last number, follow these steps:

1. Calculate the Total Sum of the 20 Numbers:
- Average of 20 numbers = 12
- Total sum = Average × Number of numbers
- Total sum = 12 × 20 = 240

2. Calculate the Total Sum of the First 12 Numbers:
- Average of the first 12 numbers = 11
- Total sum of the first 12 numbers = Average × Number of numbers
- Total sum of the first 12 numbers = 11 × 12 = 132

3. Calculate the Total Sum of the Next 7 Numbers:
- Average of the next 7 numbers = 10
- Total sum of the next 7 numbers = Average × Number of numbers
- Total sum of the next 7 numbers = 10 × 7 = 70

4. Calculate the Total Sum of the Last Number:
- Total sum of all 20 numbers = Sum of first 12 numbers + Sum of next 7 numbers + Last number
- Last number = Total sum of all 20 numbers - (Sum of first 12 numbers + Sum of next 7 numbers)
- Last number = 240 - (132 + 70)
- Last number = 240 - 202
- Last number = 38


The last number is 38.

The correct answer is B. 38.