Continuing the previous setup (classes 0-20,...,80-100; freq 17, f1, 32, f2, 19; total 120; mean 50): find f1.

Class marks 10,30,50,70,90. f1+f2 = 52. sum fi*xi = 17*10+30f1+32*50+70f2+19*90 = 170+30f1+1600+70f2+1710 = 3480+30f1+70f2. Mean 50 => sum fi*xi = 50*120 = 6000 => 30f1+70f2 = 2520. With f1+f2=52: 30f1+70(52-f1)=2520 => 30f1+3640-70f1=2520 => -40f1 = -1120 => f1 = 28.