Mohan walks from Tilak Nagar to Moti Nagar and back in a certain time at the rate of $3 \frac{1}{2} \mathrm{~km} / \mathrm{hr}$ But if he had walked from Tilak Nagar to Moti Nager at the rate of $3 \mathrm{~km} / \mathrm{hr}$ and back from Moti Nagar to Tilak Nagar at the rate of $4 \mathrm{~km} / \mathrm{hr}$., he would have taken 10 minutes longer. The distance between Tilak Nagar and Moti Nagar is
(a) $28 \mathrm{~km}$
(b) $32 \mathrm{~km}$
(c) $24 \mathrm{~km}$
(d) None of these