Grand Theft Auto: San Andreas/BMX

Get on the BMX bike that's near the half-pipe on top of the hill to start the challenge. You start with ten seconds on the clock, but you get ten additional seconds for each of the 19 coronas you touch while on the bike. Hit all the coronas to complete the challenge. Run out of time or fail to get on the bike after falling off and the challenge is failed.

Tips
The more cycling skill the better since you have to do a fair amount of bunny-hopping and skill determines your hop height. There's no hurry in completing this challenge since it doesn't have any rewards. You can up your cycling skill by doing the Los Santos Courier mission or the Tags collection mission on a bike.

Get all the easy coronas, like those in the basin and those on the ground level, first.

Some of the high coronas you can get with a bunny hop. Notice the coronas cast shadows so you can position yourself right before jumping.

There is one corona above the basin which you can get by jumping across. There is another one in the center of the half-pipe part of the basin. To get this one, ride up the side and bunny-hop backwards.

The wooden half-pipe on top of the hill has a corona at the top of each side. Ride up with a medium amount of speed and turn around as you fly off to hit them both easily.

Which leaves the one corona off the ledge on the other wooden half-pipe. To get onto this ledge, you have to ride up the half-pipe, force the front wheel down, and brake in a fluid motion. This will take some practice (which you can always do before starting the challenge). If you don't want to practise, you can just get the Fire Truck from the nearby fire station and place it in front of the half-pipe. Now jump on the truck, make sure the corona is just in front of you and jump again. This way, you won't get onto the ledge, but you will get the corona.

Rewards
Progress towards 100% Completion.