50. d) since $10 is a constant (Nelson will always pay this amount) and then 0.08m has the variable m, since the amount of times $0.08 is payed is directly related to the number of messages sent.
51. b) $200 at 15% is found by 0.85 x 200 (0.85 is the amount of the coat 'not' on sale) and the just add the $5 to that fee.
52. b) from this answer, just expand 0.7 into the two terms in the bracket