TR CO

sir while calculating labour i never put in the costs of batches directly in the formula like cost of the rate of first 1000 batch 90 X (1000)power-0.321928 i did like total time for first 1000 batches which is coming 541.9859 hours and total time for first 999 batches which is 540.6190861 hours then time for 1000th batch = 541.98 -540.6190 = 1.366814 hours then i went to calculate labour per batch which is 1.366814 X 18 = 24.6026 , why is this incorrect ? and why is examiner directly putting costs in formula ??