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What is the average back emf?

i have tried this

Flux=Magnetix Field*Area

Flux=0.06*pi*r^2

Here r=2.5cm=2.5*10^-2m

Avarage Emf=N*Flux/Time

Time=1/Speed

Speed=440 rpm=440/60=7.3333

Average Emf=Flux*150/Time

so average Emf = 0.06*phi*r^2*150 / 0.136363 = 0.1295 V

but, my answer is wrong..

please advise!