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Answer:
f1 is 20 and f2 is also 20
Step-by-step explanation:
make separate columns for class mark(x) and fx
x fx
20 1800
40 40f1
60 1800
80 80f2
100 4000
√fx= 7600+ 40f1+ 80f2
√f= 160+ f1+f2
mean=√fx/√f
50= 7600+40f1+80f2/160+f1+f2
8000+50f1+50f2= 7600+40f1+80f2
400= -10f1+30f2
3f2-f1= 40---------1⃣
also 160+f1+f2= 200
f2+f1= 40----------2⃣
equate 1⃣ and 2⃣ by elimination method.
you'll get f2= 20 and f1= 20
hope it helped :)