Because if you start with one dollar and by day 3 you get 301 dollars and option A only gives you 8 cents on day 3 you get more money by the end of the month if you choose option B.
He drove 123 miles in 2 hours. If you go 100 miles in 2 hours that means you went 50 miles in one hour right? Using the same logic we can convert 123 miles in 2 hours to 61.5 miles per hour and then multiply it by 5.