A company’s annual sales figures over the last four years are as follows: $2.6 million; $3.1 million, $5.3 million, $7.0 million. The firm’s compound annual growth rate over the period is closest to

28% 39% 56%

The solution uses 3 years to calculate the growth rate and hence the answer is 39%

I’m not understanding why itsn’t 4 years. Could someone kindly explain this to me.