Because for #3 the question states "fewer than 50 customers", when transformed to a normal distribution you can not have X<50 because this would include 50, so you need to use X<49.5 which would correspond to X<=49 for a binomial. For question #4 It is asking for a confidence interval so the problem of modelling a discrete distribution with a continuous one is of no consequence. If you don't understand continuity corrections I strongly suggest you re-read the relevant section in the notes.
