since we are given circumference, we think the formula for the circumference, which is 2(pi)r or (pi)d where r and d is the radius and diameter respectively. the smarter formula of the 2 is (pi)d because it's d we want to find. setting up our equation:
circumference = (pi)d
48 = (pi)d
dividing both sides by pi gives:
d = 15.29 millimeters
let me know if you have any questions!