Question

A Big Mac hamburger from McDonald’s contains 540 Calories.
(a) How many kilojoules does a Big Mac contain?
(b) For how many hours could the amount of energy in a
Big Mac light a 100-watt light bulb? (1 watt = 1 J/s)

The answers I got were:
a) 2259.36 kJ
b) 6.276 hours

Am I correct?

Answer 1

I agree with both answers. I note that you used too many significant figures in the answers.

Answer 2

How would you right the answers so there are enough significant figures?

Answer 3

Here is a site that does a really good job of explaining both significant figures as well as how to inject that into multiplying/dividing and addition/subtraction.
https://www.chemteam.info/SigFigs/SigFigs.html
Assuming you know how to determine the number of s.f. in a problem, then for addition/subtraction the s.f. in the answer can be no more than the least accurate in the numbers. That is
1234.11
+1234.1
-----------------------
= 2468.2
Note that the first number is known to the hundredths place but the second number is known only to the tenths; therefore, the answer may not be more accurate than the tenths. There is another rule when multiplying/dividing but that is done very well in the link I gave you above.