How would I calculate the age of a rock only given the amount of each in grams and half-life?

40K decays to 40Ar.
Half life of 40K is 1.25 billion years.
40K = 20 grams
40Ar= 5 grams

I think the formula is
Final Amount of 40K = Initial Amount of 40K * (1/2)^(time/half-life)

If I plug in the values I still don't have a value for time. And where would the grams of 40Ar fit in?

40K:40Ar is a ratio of 4:1 by mass, so final/initial would be 5/20.

.25 = (.5)^(t/1.25*10^9)
Solve for t. Time would be in the unit of years.

Thank you so much!