Based on the 2 different models, you can name the units x and y for group 1 and 2 respectively. Since both groups have the same number of students, you can equate them to be equal as shown. You will get y= x+4. The question says that 45% (0.45)of the total no. of participants are girls. The total number of participants are the sum of the both models. Equate it and you'll get another equation. Substitute the previous equation into this equation to find the values of x and y respectively. The question wishes for you to obtain the no. of boys so refer to both models and sum up the parts representing the boys. Hope this helps! (: