What are the last number in line 10 and the sum of numbers in line 12, and how do you quickly add the first 20 odd numbers?

The last number in Line $n$ is given by the formula $2n - 1$.

For Line 10, substitute $n=10$ into the formula:

The last number in Line 10 is $\boxed{\text{19}}$.

The sum of the numbers in Line $n$ is given by the formula $n^2$.

For Line 12, substitute $n=12$ into the formula:

All of the numbers in Line 12 add up to $\boxed{\text{144}}$.

We need to add the first 20 odd numbers, so $n=20$.

The sum of the first 20 odd numbers is $20^2$.

A quick way to add up the first 20 odd numbers is to square the number of odd numbers, which is $20^2 = \boxed{\text{400}}$.