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

The highest place value is in the ten thousand for both of these numbers. 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 ten thousand place, so we look at the number behind it (3) 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 4 or less, so we let it rest. So, the new rounded number is 20,000. 
Now let's look at 46,718. So, we need to round to the ten thousand place, so we look at the number behind it (6) 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 50,000.
Now, all you have to do is add 20,000+50,000, which is 70,000. Hope this helped!