Full cash flow forecast

Hello sir

I am stuck with this question relating to cash forecast from Kaplan study text,

In the near future a company will purchase a manufacturing business for
$315,000, this price to include goodwill ($150,000), equipment and
fittings ($120,000), and inventory of raw materials and finished goods
($45,000).
A delivery van will be purchased for $15,000 as soon as the business
purchase is completed. The delivery van will be paid for in the second
month of operations.
The following forecasts have been made for the business following
purchase:
(i) Sales (before discounts) of the business’s single product, at a mark-
up of 60% on production cost will be:
Month 1 2 3 4 5 6
($000) 96 96 92 96 100 104
25% of sales will be for cash; the remainder will be on credit, for
settlement in the month following that of sale. A discount of 10% will be
given to selected credit customers, who represent 25% of gross sales.
(ii) Production cost will be $5 per unit. The production cost will be made
up of:
Raw materials $2.50
Direct labour $1.50
Fixed overhead $1.00
(iii) Production will be arranged so that closing inventory at the end of
any month is sufficient to meet sales requirements in the following
month. A value of $30,000 is placed on the inventory of finished
goods, which was acquired on purchase of the business. This
valuation is based on the forecast of production cost per unit given
in (ii) above.
(iv) The single raw material will be purchased so that inventory at the
end of a month is sufficient to meet half of the following month’s
production requirements. Raw material inventory acquired on
purchase of the business ($15,000) is valued at the cost per unit
that is forecast as given in (ii) above. Raw materials will be
purchased on one month’s credit.
(v) Costs of direct labour will be met as they are incurred in production.
(vi) The fixed production overhead rate of $1.00 per unit is based upon
a forecast of the first year’s production of 150,000 units. This rate
includes depreciation of equipment and fittings on a straight-line
basis over the next five years. Fixed production overhead is paid in
the month incurred.
(vii) Selling and administration overheads are all fixed, and will be
$208,000 in the first year. These overheads include depreciation of
the delivery van at 30% pa on a reducing balance basis. All fixed
overheads will be incurred on a regular basis, and paid in the month
incurred, with the exception of rent and rates. $25,000 is payable for
the year ahead in month one for rent and rates.
Required:
(a) Prepare a monthly cash flow forecast. You should include the
business purchase and the first four months of operations following
purchase.
(b) Calculate the inventory, receivables, and payables balances at the
end of the four-month period. Comment briefly upon the liquidity
situation.

In the question they haven’t mentioned anything about the time period to be considered for calculating sales units. But the working note gives the following figures
Sales units
12k for 1 month, 12k for 2nd month, 11.5k for 3rd month, 12k for 4 month,12.5k for 5th month & 13k for 6th month.
So I don’t understand how these figures are been derived to calculate production units.