By getting a 20% discount, Calvin's daughter will have to pay only 80% or 0.80 of the adult fare. I assume that the fares given are for the to-and-fro trips and that Calvin's daughter will have to pay 80% of these fares.
to: (0.80)($42) = $33.6
fro: (0.80)($30) = $24
Adding up the fares will give us an answer of $57.6. Therefore, Calvin's daughter will have to pay $57.6 for the round-trip fare.
