The classic simplistic approach - the CAA are testing your syllabus not your knowledge. That question takes no account of the fact that the dew point changes with height! But it is what is taught at the basic level.
Do further research and you will see that the dew point change (approx 0.5 per thousand) gives a net 2.5 change per thousand feet (1000/2.5=400ft per degree) giving:
base=(temp-dew)*400.
Normally this is given as (temp-dew)*125 = base in m (metric).
Remember you are being tested on what is being taught
not necessarily the correct answer!