Maths (Speed)

50.6 km per hour

Time elapse between 18.30 and 20:30 = 20:30 – 18:30 = 2 h

Case 1: (Assuming the taxi runs faster)
Let x be the speed of bus
The speed of taxi = 50 km/h
For every hour, the difference in the distance they cover is (50 – x)
At 18:30, the bus is 3 km ahead of the taxi
At 20:30, the bus is 1.8 km ahead of the taxi
In 2 hours, the distance between them is shortened by (3 km – 1.8 km) = 1.2 km
1.2 km = 2h (50 km/h – x km/h)
1.2/2 = 50 – x
0.6 = 50 - x
x = 50 – 0.6
x = 49.4 km/h

Case 2: (Assuming the bus runs faster)
Let x be the speed of bus
The speed of taxi = 50 km/h
For every hour, the difference in the distance they cover is (x – 50)
At 18:30, the taxi is 3 km ahead of the bus
At 20:30, the taxi is 1.8 km ahead of the bus
In 2 hours, the distance between them is shortened by (3 km – 1.8 km) = 1.2 km
1.2 km = 2h (x km/h – 50 km/h)
1.2/2 = x - 50
0.6 = x - 50
x = 50 + 0.6
x = 50.6 km/h

The speed of bus can either be 49.4 km/h or 50.6 km/h, depending on whether the bus runs faster or slower than the taxi.

Comment:
At 18:30 Taxi & Bus have 3 km distance ?? The English in this question is unclear. It is assumed that “At 18:30 the Taxi and bus are 3 km. apart and at 20:30 they are 1.8 km apart” .