Step 1 : Underline the key words
A store manager calculates his store's monthly utility expenses using two expense rates r1 for the dollar cost per hour the store was open during the month and r2 for the dollar cost per hour the store was not open during the month. During November, which has 30 days, the store was open 8 hours a day, except one day when it was open for 15 hours for a special sale.
Step II Recognize the variable
Since, the store was open for 8 hours on 29 days and for 15 hours on remaining one day.
So the total number of hours the shop was open is 8(29) + 15 = 247 hours.
Therefore, the cost incurred by the shop for the number of hours it was open =247r1
There are 720 hours in the 30 days of November. The number of hours the shops remain closed = 720 -247 = 473
Therefore, the cost incurred by the shop for the number of hours it was closed =473r2
Therefore, the total expense =247r1+473r2
Hence, option A is correct.