If 4 bags of flour with an average(arithmetic mean) weight o

If 4 bags of flour with an average(arithmetic mean) weight of 15 pounds are put onto a shelf with 45-pound bags of flour,what is the average weight,in pounds,of all 8 bags on the shelf?
A.5 B.10 C.15 d.30 e.25
我算的是13.125.这个答案怎么选?