<span>Let's imagine, total number of seventh grade students is x
20% of x= 20x/100 =x/5
16 2/3% of x/5 students also go to science class
i.e; 50/3 % of x/5 = x/30 go to science & maths class.......(a)
30 students only goes to maths class( no science)
So, x/5-30 students go to science & maths class..........(b)
From (a) & (b) we can conclude
x/5 - 30 = x/30
Or x/5 - x/30 = 30
or (6x-x)/30 = 30
Or 5x/30 = 30
or 5x = 900
so, x= 900/5
x=180
So, the total number of seventh grade students which we assumed to be x is 180 .</span>