The cheapest mode of transport would be to travel by bus.
The choice between airplane and the bus would depend on the total income the individual would earn. This is known as opportunity cost.
If the individual travels by plane, it would cost $90. The income the individual would lose would be ($6 x 1) = $6.
If the individual travels by bus, it would cost $30. The income the individual would lose would be ($6 x 5) = $30. Thus, the person would choose to travel by bus.
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