An annuity with annual payments has an initial payment of 1. Subsequent payments increase by 1 until reaching a payment of 10. The next payment after the payment of 10 is also equal to 10, and then subsequent payments decrease by 1 until reaching a final payment of 1. Determine the annual effective interest rate at which the present value of this annuity is 78.60.

I can only deal with PV of the first part "increase by 1 until reaching a payment of 10" but have no idea how to do with the second part, can anyone explain it?