A train goes forward at a speed of 1m/s for 20 seconds.then it goes backward at a speed of 0.5m/s for 4 seconds.how far did the train move?

This is not a real life situation, because in real life, a train needs to accelerate to a constant speed, and then decelerate until it can stop.  In a theoretical situation, the train moves forward for 20m, and then it moves backward for 2m.  The net forward distance travelled would be 18m.

Hi Annie,
S = 1 * 20 - 0.5 * 4 = 20 - 2 = 18 [meters]
Best regards,

We are in agreement about the forward and backward distances,

but this could be a sort of "trick question" to emphasise

the difference between scalar and vector quantities.

Strictly speaking, if what was meant was how far is it from where it started

the word displacement should be used.

In this example that would be 20 - 2 = 18m

Since the question said "..how far did the train move? " it is asking about distance
which is how far an object has moved regardless of direction . (This is what we
would want to know about if we are calculating the rate of fuel usage)
In this case I reckon the answer should be 20 + 2 = 22m.
Regards - Ian

We are in agreement about the forward and backward distances,

but this could be a sort of "trick question" to emphasise

the difference between scalar and vector quantities.

Strictly speaking, if what was meant was how far is it from where it started

the word displacement should be used.

In this example that would be 20 - 2 = 18m

Since the question said "..how far did the train move? " it is asking about distance
which is how far an object has moved regardless of direction . (This is what we
would want to know about if we are calculating the rate of fuel usage)
In this case I reckon the answer should be 20 + 2 = 22m.
Regards - Ian