Nice work - especially when the answer choices are that far apart you can often get away with an estimate, which is what you did.
It was more coincidental that you got the exact same number than anything else. With division, adding the same number to the numerator and denominator doesn't necessarily keep them in proportion:
4/2 = 2
(4+2)/(2+2) = 6/4 = 1.5
The only way to keep the proportion the same is to multiply top and bottom by the same thing.
By rounding off 58 to 60 and 29 to 30, you were able to take a quick estimate, which was certainly viable, but by doing that you weren't likely to get the same exact number...that was just coincidental.