Is rounding important?

Dear Sir,

I have been practicing the revision kit for both Kaplan and BPP, Andy there is differences with the rounding, which gives us a different answer.

For example, in question 24, page 9 of BPP revision kit, the fund for construction of asset was pooled from 2 outstanding loans:

9% £15M loan
11% £24M loan

The company withdrew £6M on 1 April 20X8 and an additional £2M on 1 August 20X8.

Question: Calculate the capitalised Borrowing Cost.

A £549,333
B £411,999
C £750,000
D £350,000

The answer is A (with WA rate of 10.3% – which was rounding from the equation below).

However, if you put the whole equation in the calculator (9%*15/39 + 11%*24/39), they will give you a rate of 10.23%, and subsequently give an exact result of £545,600.

So in the exam should we use rounding instead? Because sometimes the exam may give us closely matching results.

Many thanks