15 minutes, first you need to convert everything to feet, 3yards in a foot so multiply 12 by 3 and you get 36ft, 5^-2 is 0.04, so the snail goes 0.04ft every second. Divide 36 by 0.04 to get 900, that’s the distance and if he moves like that every second then that’s also the time, divide 900 by 60 to get minutes.