The payback period occurs between years 2 and 3.
By the end of year 2, there is a shortfall of $1000000to recover the investment cost of $5000000.
In year 3, the net cash flow is $2400000,or $200000a month ($2400000/ 12 months).
The shortfall of $1000000is recovered in 5 months ($200000x 5 months).
Also, ($1000000/ $2400000) x 12 months = 5 months.
The payback period is 2 years and 5 months, or 2.42 years (2.41666 years), or 2 years and 152 days, or 2 and 5/12 years.