Jack and Bob travelled from Town F to Town G at constant speeds of 40 mph and 45 mph respectively at the same time. Some time later, they were 20 miles apart. How long had they been travelling?

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Since Jack and Bob were traveling at the same direction at the same time, distance between them would be larger and larger as time went on.

Recall the formula: Time = Distance difference ÷ Speed difference

Their speed difference: 45 - 40 = 5 mph

Their distance difference: 20 miles

Thus, the time: 20 ÷ 5 = 4 h