Final answer:
To round a number to three significant figures, we follow the rule of rounding up if the digit to the right of the desired significant figure is 5 or greater. In this case, the answer must be rounded to 424.
Step-by-step explanation:
The calculator answer of 423.6059 must be rounded off to three significant figures. To round a number to a specified number of significant figures, we follow the rule:
If the digit to the right of the desired significant figure is 5 or greater, we round up. If the digit is less than 5, we round down. If the digit is 5, we round up if the preceding digit is odd, and we round down if the preceding digit is even.
In this case, since the first digit to be dropped (in the thousandths place) is 6, which is greater than 5, we round up to 424. Therefore, the answer after rounding would be a) 424.