Final answer:
To round 77.5 to the nearest ten, you increase the tens digit since the ones digit is 5, resulting in 80.
Step-by-step explanation:
When rounding the number 77.5 to the nearest ten, we must look at the ones place digit (which is 7 in this case) to decide whether to round up or down. Since the ones place digit is 5 or above, we round up. Therefore, 77.5 rounded to the nearest ten is 80.