The rate for the first is 1 job / 35 minutes and for the second is 1 job / 15 minutes. So combined we get
r = 1/35 + 1/15
3×5×7r = 3 + 7 = 10
r = 10/(105) jobs per minute
We're interested in 1/r
1/r = 105/10 = 10.5 minutes per job
Answer: 10.5 minutes