MA[labour efficiency variance]

Hello, Sir! Could you explaing how they get 90% step by step, please? The answer which is given by BPP writers isn't so clear. What does 90% mean in the labour efficiency variance answer of this question? Question XYZ Co is planning to make 120,000 units per period of a new product. The following standards have been set. Per unit Direct material A 1.2 kg at $11 per kg Direct material B 4.7 kg at $6 per kg Direct labour: Operation 1 42 minutes Operation 2 37 minutes Operation 3 11 minutes Overheads are absorbed at the rate of $30 per labour hour. All direct operatives are paid at the rate of $8 per hour. Attainable work hours are less than clock hours, so the 500 direct operatives have been budgeted for 400 hours each in the period. Actual results for the period were: Production 126,000 units Direct labour cost $1.7m for 215,000 clock hours Material A cost $1.65m for 150,000 kg Material B cost $3.6m for 590,000 kg Answer Labour efficiency variance 126,000 units should take (x 1.5 hrs) 189,000 hrs but did take (215,000 hrs x 90%) 193,500 hrs 4,500 hrs (A) Standard rate per hour x $8 $36,000 (A)