Prolog retrieving data from a knowledge base and arithmetic operation

Question

Hi this is a question I ran into when I was going through some question sets. The knowledge base is given with the following information,

boat(Boat_ID, Boat_name, color,fare_per_passenger)

The question says to write a procedure to do the following,

to obtain the boat identification number together with the fare per passenger increased by 5%.

I wrote the procedure as,

boat(BID,_,_,F), F is F*1.05.

Prolog keeps returning me with a message saying false any idea?

Answer 1

The answer to this question was,

boat(BID,_,_,F), Nfare is F*1.05.

the issue was with using the same variable for the arithmetic operation as well.

Credit to Daniel Lyons for mentioning it.