Vin(According to virtual ground concept)What is the voltage at the junction of R1 and R2?
I found the problem with my approach.But how is Vout divided by R3 and R4 since r1 and r2 are parallel connected, same thing with Vin
R1 and R2 are NOT parallel connected!But how is Vout divided by R3 and R4 since r1 and r2 are parallel connected, same thing with Vin