First of all this is a ratio topic and to solve it one needs to know that ratios are fractions so to solve it we will find first the sum of the fractions which is 3+1 and =4. Fully recognising through the question that the fractions or relation of the women to the men in the classroom is in a 3:1 ratio, we should multiply the fraction with the total number of people in the class which bring forth the result as 18!
