Girish Shambu and William Reisel are teammates at a discount store; their new job is assembling bicycles

Question:

Girish Shambu and William Reisel are teammates at a discount store; their new job is assembling bicycles for customers. Assembly of a bike has a learning rate of 90%. They forgot to time their effort on the first bike, but spent 4 hours on the second set. They have 6 more bikes to do. Determine approximately how much time will be ( was) required for: a) the 1st unit
b) the 8th unit
c) all 8 units