In this problem, we are looking for the doubling time of the wage of the Toyota automobile assembly line workers. Initially, their wage is $6.70 and by the end of 15 years, their salary is now $29.59.
What we can find here first is the rate of the increase of the salary of the workers. We can use the equation

We are looking for the value of r. We have to derive an equation solving for r. We have

Now, we solve for the value of r given y = 29.59, y0 = 6.70, and t = 15 years. We get

The doubling time is computed as

Substitute the value of r on the equation above and solve, we get

Hence, the doubling time for this problem is equal to 7 years.
Answer: 7 years