Chemistry question rate constants?

kt= 2,303 log a/a-x a and a-x can be in moles , atoms or in weights. o.0231 x 60 = 2.303 log a/40 . 0.60=log a/40 . log4=log a/40. 4=a/40. a=160 grams . a is the initial weight of the radio active element. radio active disintegration of the radioactive element is first order decay.