Having problems understanding learning theory math. Could you please show me all the calculations to solve the following: To estimate how long it would take you to assemble the fourth bookcase, we can use the following formula:

time = a * (b ^ n)

where a is the time taken to assemble the first bookcase, b is the percentage reduction in time for each doubling of quantity (calculated as 1 - r), and n is the number of bookcases to be assembled. Using this formula with a = 100 minutes, b = 0.9, and n = 4, how long would it take to assemble the fourth bookcase?