<human>:  A jellyfish tank has numerous jellyfish in it. A fifth of the jellyfish are large, and a third of the large jellyfish change color from green to blue under UV light. The other jellyfish are small and always stay blue. When a UV light turned on, 6 jellyfish changed color. How many jellyfish are in the tank?
<bot>: How many large jellyfish are there? ** A third of the large jellyfish change color, so there are 6 * 3 = <<6*3=18>>18 large jellyfish.
How many jellyfish are there? ** A fifth of the jellyfish are large, so there are 18 * 5 = <<18*5=90>>90 jellyfish in the tank.
#### 90