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The question is:

A child of mass 30kg is standing in a lift which is descending. The force exerted on the floor of the lift by the child has magnitude 270N. Find the magnitude of the acceleration of the lift and state whether the life is speeding up or slowing down.

I've tried 1) splitting the objects to get a simultaneous equation, but there were too many unknowns 2) treating the objects as one and getting lost.

Could someone give me some pointers on what I should try to do.

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    May be $270 N=30 kg a_{total}\frac{m}{s^2}$ and so $a_{total}=g+a_{lift}=9\frac{m}{s^2}$ and thus $a_{lift}=-0.81\frac{m}{s^2}$ and the lift is slowing down? – 2017-01-19
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    what do you mean by g? gravity? – 2017-01-19
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    the gravitational accelleration $g=9.81...\frac{m}{s^2}$ but I´m not sure bnosnehpets answer seems more professional to me – 2017-01-19

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Use Newton's Second Law. Observing from an inertial reference frame outside the lift, the force on the person inside the lift is: $$ma = mg - R$$ where $R$ is the reaction force from the lift, $m$ is the mass of the person, $a$ is the acceleration of the lift and $g$ is the gravitational acceleration.