Coin Slots is the 28th stage of Advanced Mode in Super Monkey Ball 2 (67th in Super Monkey Ball Deluxe), the 8th stage of Inside A Whale in Story Mode (13th in Super Monkey Ball Deluxe), and the 127th stage of Ultimate Mode without continues (107th with continues).
This stage consists of a large basin with its sides cut out, in which there are holes along its bottom. There are goals bouncing around it, occasionally and partially touching these slots until they go in.
There are nine goals in this stage, eight blue goals and one green goal.
As you go farther away from the starting platform, you'll notice that the goals are going slower as you go back. You may want to try one of the farther back ones, so you will more likely have an easier time beating the stage. Approach the ribbon at the right time at a slight speed, and go through to beat the stage.
This goal is considered very difficult to roll through. For the challenge mode version, pause buffering (pause and un-pause very quickly) until the timer reads 57.05, and then going forward will work. You may want to change the timing a bit depending on how fast you go. Experimentation in Practice Mode is recommended for this strategy.