b= ln(0.9)/ln2 = -0.152
a = 10 min
n = 50
Avg .time taken to assemble n table fan = a * n^b
So, Avg time taken to assemble 50 table fan = 10 * 50^(-0.152) = 5.518 mins (approx)
Therefore, total time taken to assemble 50 table fan = 50*5.518= 275.9 minutes
Similarly, Avg time taken to assemble 49 table fan = 10 * 49^(-0.152) = 5.535 minutes
Therefore, total time taken to assemble 49 table fan = 49* 5.535 = 271.215 minutes
So the time taken to assemble 50th table fan would be roughly 275.9 - 271.215 = 4.685 minutes