Mr. Prasad travelled equal distances at speeds of $2 \mathrm{~km} / \mathrm{hr}, 4 \mathrm{~km} / \mathrm{hr}$, and $6 \mathrm{~km} / \mathrm{hr}$, and took a total of 55 minutes to complete. Find the total distance he travelled, in $\mathrm{km}$.
(a) 2
(b) 1
(c) 4
(d) 3