How 870.0 contain 4 significant digits?

Answer 1

Trailing zeros in a number containing a decimal point are significant. For example, 12.2300 has six significant figures: 1, 2, 2, 3, 0 and 0. The number 0.000122300 still has only six significant figures (the zeros before the 1 are not significant). In addition, 120.00 has five significant figures since it has three trailing zeros.

Answer 2

The fact that there is a trailing zero and the number is not given as 870 is indicative that the number is accurate to the nearest tenths, not just the nearest integer. Therefore, it has four significant figures.