A man normally takes 5 hrs to travel at a certain speed from city A to B. One day, he increases his speed by 4 km/h and finds that the journey...

Let the distance from A to B be
D.

==> The time is T1 = 5
hrs.

==> Then, let the speed be S1 = D/T1 = D/
5

==> S1 = D/
5

==> D
= 5*S1 .............(1)

After increasing the speed ( the
distance remains the same.)

==> S2 = S1 +
4 

==> T2 = T1 -
(0.5)

==> D = S2* T2 = (S1 + 4) * (5 -
1/2)

==> D = (S1 +
4)(9/2)

==> D = (9/2)S1 +
18

But D = 5S1

==> 5S1
= (9/2)S1 + 18

==> (5-9/2) S1 =
18

==> (1/2) S1 =
18

==> S1 = 36 km/
h

Then, the normal speed is 36 km /
h