Step 1 : let no of boys in each group are x and girls in each group are y; which makes n(x+y)=35. Step 2: Find hcf, which is 5. That is n=5. Step 3: boys = 20; 20/5= 4; in each group 4 boys are there. Step 4: girls =15; 15/5=5; in each group 3 girls are there.