Bheja fry question

 This bheja fry question I heard on TV.

A chicken costs Rs 10.

A honeybee costs Rs 30.

A spider costs Rs 40.

How much does a cat cost ???

To be honest, I thought it is Rs 20  !!!!

Because only 20 was missing..... 

10, 30, 40 were already there  !!!!!

And it is the right answer  !!!!!

But mine was a guess work..... 

Actual logic is

A chicken has 2 legs. Cost is Rs 10.

A honeybee has 6 legs. Cost is Rs 30.

A spider has 8 legs,. Cost is Rs 40.

This means each leg costs Rs 5.... 

We all know that a cat has 4 legs.......

According to logic, 

Cat must cost  5   ×  4    =   20  Rs.    !!!!