Only want to bet on a firm fav - how do I put this in a condition

Hi,

Sorry for the basic questions, how do I say only bet on the fav if there a 1.5 gap to the second fav.

For example 1 fav is 1.40 and second fav is 2.90 , so I want to back the first fav as it’s a firm fav (1.5 away from second fav).