Puzzle ZSSR 

Workings

Hint
How many mice does one cat catch?

Answer
Six mice.

  1.5 cats catch 1.5 mice in 1.5 days.

so

  1 cat catches 1 mouse in 1.5 days.

  1 cat catches 2 mice in 3 days.

Therefore

  3 cats catch 6 mice in 3 days.