Hi guys i just started this new unit, have a few question not having learned about logarithms, What would the formula look like?

1) It is estimated that 30% of a certain substance decays in 20 hours. what is the half life of the substance.

-- This is what i do to solve Y= AB^x- 50%=100%(100-30%=70 or .7%)^T = .5%=(.7)^T What is T in this case and how do i incorporate the 20 hours into this question? as well did i use the data given in the question correctly?

Thanks in advance.