1.
The numbers on each board belong together.
2.
What do the numbers have in common?
3.
The numbers all have a common divisor.
4.
The three symbols on the beam over the fish indicate the relevant final answers.
5.
Double triangle symbol: common divisor of the series is seven. Triangle and plus symbol: common divisor of the series is eight. Snail symbol: common divisor of the series is three.