Thanks for your answer. All my workings are in the correct order except for QR, I have rounded off my answer to 15.7 (3 s.f.) whereas your is 15.65 (4 s.f). Therefore, this had an impact on my final answer. I am wondering what is the correct way, school says that we must always round to 3 sf. Did I misunderstood some where...

Ahh alright i understand. If you keep to 3sf, then you're not wrong. But do remember to keep to the same decimal place as well. Because for this example 15.65 is a value with two whole numbers so naturally when you round off to 3sf it will go to 15.7. Its best if you keep the decimal places the same as it will give a more accurate result. Hope my explaination help you.

Thank you Sir for your advice. I did kept all my answers to 3 sf however the answer key on the book shows 7.50m whereas I derived at 7.55m wondering will I lose marks in exams for this.

Dont worry about it. Just remember that once you see decimals keep all your answers all in the same decimal numbers. Other than that you can never go wrong. Good luck.

Thanks a million.