Amy, it depends on where you're staying, but if you're staying downtown, then it's good enough.
Seattle is primarily a bus-only town but there's light rail from the airport to downtown. Here's the
overview map of the busing routes in the relevant area.
You'll also want to download the app OneBusAway, which is useful when you're waiting for buses, since they are notoriously off-schedule, considering they use the same roads as all traffic. That said, weekend mornings are probably more reliable.
Assuming staying downtown, let's take a look at what you'd need for SART.
Saturday
Airport to Downtown: Link Light Rail. The south terminus of the line is at the north end of the airport. The north end of the line is downtown. Easy peasy.
Downtown to Time Trial (Bathhouse Theater in Green Lake Park): Take the
RapidRide E line from downtown to the stop at N 76th Street & Aurora Ave. The
current schedule runs through September 25th, but it looks like a 20-minute ride at 15-minute frequency on Saturday mornings. Then it's a
350m walk to the parking lot where we're staging.
Time Trial to Round 1 (Woodland Park): Honestly, the best bet is to either catch a ride from someone, or just walk. It's
2.4km and most of it is along a pedestrian path along the lake, and through Woodland Park (you could get there with a slightly shorter path, but that would take you through the competition area!). Alternatively, you could walk 350m back to the RapidRide E stop, take it south to the N 46th Street stop, and
walk 560m to the staging area.
Round 1 to Rounds 2 & 3 (North Seattle College): Two options here. Walk 560m to the RapidRide E stop at N 46th Street & Aurora Ave, take it north to the stop at N 95th Street & Aurora Ave, then
walk 880m to campus. Buses
every 12 minutes this time of day, with about a 7 minute ride.
Or
walk 750m to the 16 Line stop at N 45th Street & Stone Way, and take the 16 Line straight to the North Seattle campus. Should be running
every 20 minutes on Saturdays, and it's about a 20-minute bus ride.
Rounds 2 & 3 back to Downtown: The 16 Line departs from the campus and heads straight downtown. Every 20 minutes, about a 35 minute ride. There are two other possibles here: The RapidRide E (with walk) and the 40 Line, but one has a walk, the other takes a more roundabout route back downtown.
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Sunday
Downtown to Shoreline Community College (Rounds 4 & 5): Take the 5 Local Line. It goes from downtown and ends at the College. Departs
every 30 minutes from downtown, about a 35-minute ride.
This is also your best bet to get back to downtown, too.