Its easy.
See
A) This cannot be the answer as the highest number of cars sold in one day can be as high as 300,
Lets say 300, 100, 75, cars are sold in any 3 days. so number of cars sold on friday can be less than or greater than 50. we can't determine

B)77 is the highest number of cars sold in one day. Cars left = 500-77 = 423 for 6 days.
Even if we divide them equally 70 cars will be sold in a day.
For this we can be sure that atleast 50 cars will be sold on Friday

Hence B

rahulvsd wrote:At a certain auto dealership 500 cars were sold last week. If a different number of cars were sold on each day, did the dealership sell at least 50 cars on Friday of that week?

1. On Monday, the dealership sold 75 cars, which was the third highest number of cars sold that week.

75 is the third highest. So top value can be 300, 125, 75, 25, 25, 25, 25 - atleast 50 cars on friday ? don't know.
77, 76, 75, 74, 73, 72, 53 = total 500 cars. Be it any day 50 cars will be sold. So friday is one among them. Yes.

INSUFF

2. On Saturday, the dealership sold 77 cars, which was the highest number of cars sold that week.

77, 76, 75, 74, 73, 72, 53 = total 500 cars. Be it any day 50 cars will be sold. So friday is one among them. Yes. SUFF.

Always consider highest and lowest range of values when faced with these sort of problems.

IMO B.