solution:
T1 = time required for first unit = 25 minutes
T2 = time required for 2nd unit = 20 minutes
T3 = time required for 3rd unit = 17.55 minutes
Tn = time required for nth unit
Learning rate from 1st to 2nd unit = Time required for 2nd unit/time required for 1st unit = L2/ L1
T2/ T1 = 20/25 = 0.80