Remember this: You won't have access to a calculator during the nav exam, and the exam is multiple choice. So you only need to guesstimate the correct answer with about 2-3 digits precision, and pick the right guesstimate from the four choices. Therefore when practicing for the exam, also don't use a precise calculator but try to guesstimate.
Furthermore, the CAA wants to test a few things with these questions:
1. You have a knowledge on how the worlds coordinate system works (latitude/longitude) and are able to perform some fairly basic calculations on this. Without a calculator, practically speaking the only calculations you can do are along a meridian (N <-> S, possibly crossing the North or South pole or the equator), or along the equator (possibly crossing the 0 or 180 degree meridians). Anything else requires fairly complex trigonometry and thus a calculator.
2. You know that 1 degree of latitude is 60 NM, or 1 minute of latitude is 1 NM. (Note that one degree along the equator is near, but not exactly 60 NM due to the earth not begin a perfect sphere.)
3. You know that 1 NM is 1.8 KM (rounded).
All three of these knowledge points can be combined in a variety of ways into a question but the question you quoted is the most typical example.
Armed with that knowledge, you already did a very good job and have nothing to worry about on the exam. Really your only mistake was that 29°45' (29 degrees 45 minutes) is 29.75° (29 and three quarters degrees), as already pointed out.