By Joshua Freed -- AP Business Writer

MINNEAPOLIS (AP) — Summer is almost here and soon school will be out, but there's one big math problem left — whether to fly or drive to that vacation spot.

It's a tricky equation, with many variables. Gas prices. Air fares. Are you going alone or with a spouse and kids? Stop at a motel, or drive all night? Car rental?

Financially speaking, the most important figure is the number of travelers. Destinations that might make sense for a couple to fly to can become prohibitively expensive for a family of four or more.

If you choose to fly, remember you'll pay for more than just the air fare. Unless you're flying Southwest Airlines or have elite frequent-flier status, there's a good chance you'll pay baggage fees. Even with one bag per person, a $15 bag fee would add $120 to the round-trip cost for a family of four. You'll also need transportation to the airport and possibly a rental car when you get where you're going.

Of course, taking the family car isn't free either.

Gas prices have crept higher, and now average $2.23 a gallon, according to the auto club AAA. A good way to figure the total gas cost is to use the AAA's gas price calculator, at fuelcostcalculator.com. It can figure out the miles between cities, factor in the model car you'll drive, and then do the math based on today's gas price. It's about as specific a gas calculation as you can get.

Don't forget to plan on food and possibly a motel along the way. Those could tip the balance back in favor of a flight, for some trips.

Besides financial variables, there's your sanity to consider, said Sally Black, who runs VacationKids.com, which caters to traveling families. Do you like long drives? Is there a DVD player in the minivan, or will you tell the kids to play the "Alphabet" game off billboards?

"Is it easier," she said, "to spend four hours with kids in an airplane, or 12 hours with kids in a minivan?"

She said she's seen the full range of penny-pinching in traveling families. Some try to sneak a 4-year-old on a flight as an infant to avoid buying a ticket. One family she knows paid to bring the au pair, and the au pair's boyfriend.

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