Find it Out !!!

The two persons walk in the same direction. Therefore, their relative speed = 5-4=1 km/h.

Distance covered in 1 round on the circular path = 35 km.

So, They will meet after $\dfrac {35} {1}$ = 35 hours. Hence, the answer is 35 hours.

Their relative speed = 5-4 = 1km/h

Distance covered in 1 round = $\large \dfrac {35}{1}$ = $35$ hours

Because the path is circular, the position of A and B on the track after $x$ hours is given by $4x \pmod{35}$ and $5x \pmod{35}$ , respectively.

When A and B are at the same position, $4x \equiv 5x \pmod{35}$ .

So, $4x - 5x \equiv 0 \pmod{35}$ .

Hence, $-x \equiv 0 \pmod{35}$ .

The smallest $x > 0$ that satisfies this condition is $x = 35$ hours.