Formula convert Odd BACK to LAY

[Berimbau]

Someone can help me to have formula formula for calculate and convert odds from BACK to LAY.
Example i have BACK at 1,20 and LAY for this is 6
What's the formula to have 6 ?

[jamesedwards]

Someone can help me to have formula formula for calculate and convert odds from BACK to LAY.
Example i have BACK at 1,20 and LAY for this is 6
What's the formula to have 6 ?

(1/(1.2-1))+1