Estimate 23,945 + 46,718 by rounding to the second highest place value.

The 2nd highest place value is in the thousands place for 23,945 and 46,718. Let's round the two numbers first, and then we will add them.
Let's round 23,945. So, we need to round to the thousands place, so we look at the number behind it (9) and act accordingly. 
Remember:
5 or more? Raise the Score 4 or less? Let it Rest
We are looking at the bolded number: 23,945. This number is 5 or more, so we raise the score. So, the new rounded number is 24,000. 
Now let's look at 46,718. So, we need to round to the thousands place, so we look at the number behind it (7) and act accordingly. Look at the bolded number: 46,718. This number is 5 or more, so we raise the score. The new rounded number becomes 47,000.
Now, all you have to do is add 24,000+47,000, which is 71,000.
Hope this helped!