mechanics help

a girl of mas 55 kg is standing in a lift which is moving with an upward acceleration of 0.15 ms^-2. the force exerted on the floor of the lift by the girl has magnitude "R" newtons. Find the value of "R"

I guess, I just need to know how do u get value of R (with full steps)
thank you..

Consider the forces acting on the girl. What's downward force? The upward force would be $R$. So then use $F=ma$ to determine R.

isn't the downward force R ( it is said so)?

Yes, but note that the girl exerts the same amount of force onto the floor (which is $R$), as the floor exerts onto the girl in order to cancel each other - otherwise the girl would float up or go through the floor, which is nonsensical. This force acts up (for the girl) and down (for the floor). Hence, we can use the dynamics of the girl to figure it out.