standard costing and variance

Q Ltd operates a system of standard costing and in respect of one of its products which
is manufactured within a single cost centre, the following information is given.
For one unit of product the standard material input is 16 litres at a standard price of
£2.50 per litre. The standard wage rate is £5 per hour and 6 hours are allowed in which
to produce one unit. Fixed production overhead is absorbed at the rate of 120% of
direct wages cost.
During the last four-week accounting period:
The material price variance was extracted on purchase and the actual price paid was
£2.45 per litre.
Total direct wages cost was £121,500.
Fixed production overhead incurred was £150,000.
Variances Favourable Adverse
£ £
Direct material price 8,000
Direct material usage 6,000
Direct labour rate 4,500
Direct labour efficiency 3,600
Fixed production overhead expenditure 6,000
You are required to calculate for the four-week period:
(i) budgeted output in units;
(ii) number of litres purchased;
(iii) number of litres used above standard allowed;
(iv) actual units produced;

Workings
(a) (i) Budget overhead cost = £150,000 – £6,000
= £144,000
Budget cost/unit = 120% (6 hrs × £5)
= £36
Budgeted units =
£36
£144,000
= 4,000
(ii) Material price variance = £8,000 (F)
Price variance/litre = £0.05 (F)
Quantity purchased =
05.0£
,8£ 000
= 160,000 litres
(iii) Usage variance = £6,000 (A)
Standard price = £2.50/litre
Excess usage =
50.2£
,6£ 000
= 2,400 litres
(iv) Actual wage cost = £121,500
Total wage variance = £900 (A)
–––––––
Standard cost = £120,600
–––––––
Standard cost/unit = 6 hours @ £5 = £30
Actual output =
£30
£120,600
= 4,020 units

problem
I understand how the 4020 actual units have been calculated, however if inserted to calculate a variance, say direct material usage variance it will not give -6000 as per the question.

material usage variance=((16*4020)-160000)*2.5= -239200
while the question says its -6000

Another way to calculate the variance is using the material usage variance
material usage variance= (160000- std qty)*2.5
-6000=(160000-std qty)*2.5
std quantity= 157600 litres
no. of units= 157600/16=9850 units.

However if 9850 units is used then labour rate variance will not be -4500

either way, the units calculated are different and non fit into each other.
Please i hope you can assist me understand if im going wrong anywhere.

thank you.