Apply Power Law of Practice to calculate the time taken to meet the usability criteria when the first trial took 15 seconds, how long will it at the 75 trials?

Power Law of Practice - Tn=T1na - Tn to complete the nth trial is T1 on the first trial times n to the power -a; a is about .4, between .2 and .6 - Skilled behavior - Stimulus-Response and routine cognitive actions - Typing speed improvement - Learning to use mouse - Pushing buttons in response to stimuli - NOT learning