Round 78.4 to the nearest whole number.

To round 78.4 to the nearest whole number, you look at the digit in the tenths place, which is 4 in this case. The rule for rounding tells us that if the tenths place is less than 5, we round down to the nearest whole number. Since 4 is less than 5, we take the whole number part of 78, which is 78, and do not round up. Therefore, rounding 78.4 gives us 78 as the final answer. This process clearly illustrates the application of the rounding rule.