(128 oz) * (2 oranges)/(5 oz) * (1 bag)/(11.5 oranges) * $3.49/bag ≈ $15.54
Note that we have assumed 11.5 oranges per bag, and that partial bags are available.
If Chantal must buy whole bags of oranges, 5 bags will be needed and the cost will be $17.45.