78.9 + 890.43 - 21 = 9.5 x 10^2 what is the answer rounding with significant figures? i thought it would be 9.5 since 21 has two significant figures but it’s not.

948 or 9.48 x 10^2

There are two sets of rules for significant figures

•One set for addition and subtraction

•Another set for multiplication and division

You used the set for multiplication and division.

This problem involves addition and subtraction, and the rule is

The number of places after the decimal point in the answer must be no greater than the number of decimal places in every term in the sum.

Thus, we have

78.9

+890.43

-21.

= 948.33

The "21" term has the fewest digits after the decimal point (none), so the answer must have no digits after the decimal point.

To the correct answer is 948 = 9.48 x 10^2. It has three significant figures.

that's what i got you can count the number of atoms