\text{Given:}
t_J = t_C - 21
\frac{1}{t_J} + \frac{1}{t_C} = \frac{1}{10}
1. Substituting into combined work formula:
\frac{1}{t_C - 21} + \frac{1}{t_C} = \frac{1}{10}
2. Multiplying by \( t_C(t_C - 21) \):
t_C + (t_C - 21) = \frac{t_C^2 - 21t_C}{10}
3. Combining and simplifying:
2t_C - 21 = \frac{t_C^2 - 21t_C}{10}
20t_C - 210 = t_C^2 - 21t_C
4. Solving quadratic equation:
t_C^2 - 41t_C + 210 = 0
t_C = \frac{41 \pm 29}{2}
5. Finding \( t_C \):
t_C = 35 \, \text{(valid solution)} \qquad t_C = 6 \, \text{(invalid)}
\text{Therefore, the time it takes each to complete the job alone is:}
t_C = 35 \, \text{hours}
t_J = 14 \, \text{hours}