XDF数学PS159？

A corporation with 5,000,000 shares of publicly listed stock reported total earnings of $7.20 per share for the first 9 months of operation. During the final quarter the number of publicly listed shares was increased to 10,000,000 shares, and fourth quarter earnings were reported as $1.25 per share. What are the average annual earnings per share based on the number of shares at the end of the year?

Answer: $4.85         

如果按照答案的话式子应该是（7.2×5000000+1.25×10000000）/10000000，但是题目说increased to 10,000,000 shares,那么后3个月不是应该算增加的5000000吗，搞不懂唉！