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.

considering the question as per the attachment

here we can see that the radon nuclide has 222 as the atomic mass number and 86 as the atomic number.

when it undergoes nuclear fission it gives polonium which has atomic mass number as 218 and atomic number will be 84 along with a helium nucleus which has 4 atomic mass number and 2 as its atomic number.

222 = 218 + 4 (atomic mass number)

86 = ? + 2; therefore ? = 86-2 = 84.

the atomic mass number of polonium will be 84.