A man standing in a lift holds a spring balance with load of 5 kg suspended from it.What would be the reading of the balance when the is descending with an acceleration of 3.8 m/s^2.my answer is 30 N,but in book it is given 3.06 kg.They have divided 30 by 9.8 but reading on balance should be 30,so tell me which is right.

I like your answer better than theirs but both are "right." You are correct that the force on the balance is indeed 30N. But balances are designed to report the mass that under nomal earth gravity acceleration constant 'g' gives the force they measure. If you place a 20 Kg mass on a scale the scale will report 20 Kg, not 20 x 9.8 = 196 N. So when people say something "weighs" 20 Kg it's a misnomer - the object has a mass of 20 Kg but weights 196N - assuming the object is under normal earth gravity.

In this case the scale feels 30N, which is the force produced by a mass of 3.6 Kg under normal earth gravity of 9.8 m/s^2. Hope my explanation is clear.