A gear ratio is the ratio of the number of rotations of a driver gear to the number of rotations of a driven gear. A colon is often used to show a gear ratio: gear ratio = rotations of a driver gear : rotations of a driven gear.
In this case the ratio is 30:60 or 1:2 so the idler gear rotates twice for each rotation of the driver.
Also you can think of it like Work Done = Force x distance.
As Circumference is Pi and diameter the larger gear has twice the distance to travel compared to the idler.
Just a bit of fun really
