Here's why:

* Trailing zeros without a decimal point are ambiguous. They could be placeholders or they could be significant.

* To determine the number of significant digits in 780, we need to consider context. If 780 is a rounded number, it has two significant digits. If it is an exact count, it would have three.

Without further information, we assume the trailing zero is not significant. This means only the 7 and the 8 are significant.