Mr. Boll can pick a bale of cotton by hand in 5 days. If his son joins him in the work, they can pick a bale in 3 days. How long would the son take to do the job?

Okay so first you know that the dad can do it in 5 days and together they can do it in 3. so that's 1/5 for the dad and 1/3 together and x for the son. so the equation would be 1/5 + 1/x = 1/3. Then subtract 1/5 to the other side and that would be 2/15, then multiply the x to 2/15 so the equation now would be

1 = (2/15) x. The final answer is 15/2 which is 7 1/2 days.