Future value of an annuity

A client plans to send a child to college for four years starting 18 years from now. Having set aside money for tuition, she decides to plan for room and board also. She estimates these costs at $20,000 per year, payable at the beginning of each year, by the time her child goes to college. If she starts next year and makes 17 payments into a savings account paying 5 percent annually, what annual payments must she make?

Could anyone explain me why this problem is ordinary annuity not annuity due? I thought it was annuity due problem guessing from payable at the beginning of each year.
Thank you in advance!

The 70,919 is the PV at time 17, which is the exact same point in time that 25.840366X applies. You could calculate the PV of an annuity due at time 18, but then you have to accumulate the 25.840366X with one year of interest.