A bakery measured the mass of a whole cake as 0.870 kg. A customer bought one slice of the cake and measured the mass of the slice as 0.1151 kg. What is the mass of the remaining cake, without the one slice? Express your answer in kilograms using the appropriate number of significant figures.

The mass of the remaining cake where one slice was already taken is the difference between its initial mass and the mass of the slice taken. That is,

m = 0.870 kg - 0.1151 kg = 0.7549 kg

The proper number of significant figures of the operation is the lowest number of the decimal places. Thus, the final answer should be 0.755 kg.

mass of a whole cake = 0.870 kg

the mass of the slice as 0.1151 kg

Hence,

0.870 - 0.1151 = 0.755 and NOT 0.7549

(the tricky bit is that you can only give the answer to 3. sig figs)