When kettle is full of water it has a mass of 2900 grams.
when the kettle is half full of water it has a mass of 2050 grams.
what is the mass of the kettle when it is empty?

Ok what do we know.

when the kettle is full its 2900g
when its half full is 2050g

that means that half the weight of water in the kettle was 850g so the other remaining half has to be 850g

850+850=1700

1700g is the weight of the water
but we want to know the weight of the kettle when its empty so you had 2900g when the kettle was full lets say you poured all 1700g of the water out you would be left with...

2900-1700= 1200g
this means the kettle when empty weighs 1200g