Methods for calculating the payback period

Hi, I wondered whether someone could help me work out why two different methods of calculating the payback period have been used that give 2 different answers.
In the BPP P&R pg36, Q92 is as follows:
NW Co is considering investing $46k in a new delivery lorry that will last 4 years, after which time it will be sold for $7k. Dep’n is SL. Forecasting profits/(losses) to be generated by the machine are below:
Yr1 $16.5k
Yr2 $23.5k
Yr3 $13.5k
Yr4 ($1.5k)
Assuming operational cash flows arise evening over the year, what is the payback period for the investment.
Answer:
Dep’n = ($46,000-$7000)/4 = $9,750

Time 0 Investment ($46,000)
Time 1 Cash flow $26,250 (Profit in Yr1 $16.5+Depn $9,750)
Cumulative CF ($19,750)

Time 2 Cash flow $33,250
Cumulative CF $13,500

Therefore payback period = 1+($19,750/$33,520) = 1.59 * 12 = 19 months, i.e. 1 year 7 months.

When I apply this method to the following question the answer is wrong and the method used to calculate the correct answer is also completely different:
Q98

A project has average estimated cash flows of $3k/yr with an initial investment of $9k.
Depn is SL with no residual value and the project has a 5 year life span. The company has a target return on ROCE of 15% and a target payback period of 2.5. ROCE is based on initial investment.
Under which investment appraisal method, using the company’s targets, will the project be accepted?

The answer is none.
However when I use the method above to get the answer for payback it differs from BPP:

Depn = $9,000 / 5 = $1,800

Time 0 Investment ($9,000)
Time 1 Cash flow $4,800
Cumulative CF ($4,200)

Time 2 Cash flow $4,800
Cumulative CF $600

Payback period = 1+($4,200/$4,800) = 1.875 * 12 = 22.5 months, therefore 1yr 10.5 months.

BPPs method:
9,000 / 3000 = 3 years

What I am missing in Q98 that suggests I should be doing the simple calc of $9k/$3k?

Thanks