There are two ways to do this problem.

First way.

DISTANCE = RATE × TIME

After 2 hours, it has gone half-way.

DISTANCE = 40 km/hr × 2 hr
DISTANCE = 80 km

Since half-way is 80 km, the whole way is 2 times that.

Answer = 2 × 80 km = 160 km

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Second way.

DISTANCE = RATE × TIME

Since after 2 hours, it has gone half-way,
then after 4 hours (which is 2 times 2 hours),
it will go the whole way:

DISTANCE = 40 km/hr × 4 hr
DISTANCE = 160 km

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Edwin