To find the expected number of dog owners who brush their dog's teeth, we can multiply the total number of dog owners (639) by the percentage that brush their dog's teeth (18% or 0.18).
Expected number of dog owners who brush their dog's teeth = 639 x 0.18
= 115.02 (rounded to the nearest whole number)
So, we can expect about 115 dog owners out of 639 to brush their dog's teeth.