Variances help

Dear tutor,

I have followed exercises on variances and i am practicing. I have no problems with simple variances calculations, but strugling with more complex exercises. For example I don’t understand this two different methods of calculating actual units produced in two different exercises:

EXERCISE 1
Total actual direct material cost 2,400
Add back variances: direct material price (800)
direct material usage 400
Standard direct material cost of production = 2,000

Standard material cost per unit $10
Number of units produced (2,000 / $10) = 200

EXERCISE 2

Looking at this solution, I don t understand the reason why they subtract-add only one variance (usage variance) to come up with standard cost of actual production, and not both price and usage variange, when calculating actual production. Shouldn’t be the reverse of the above exercise?

Standard cost per unit = 10.5 litres × $2.50 per litre
= $26.25 per unit
Standard cost of actual production = standard cost + usage variance
= $(12,000 litres × $2.50) + 1,815
= $(30,000 + 1,815)
= $31,815

Actual production = standard cost of actual production/standard cost per unit
= 31,815/$26.25
= 1,212 units

thank you very much for your help.