Abox holds 18 chocolates and has a gross mass of 310 g. each chocolate weighs the same. when 8 chocolates have been eaten, the box and the remaining chocolates together have a mass of 210 g. what is the mass of each chocolate?

Let 
x-----------> the mass of each chocolate
y------------ the mass of the box

we know that
18x+y=310-----> equation 1
10x+y=210------> equation 2

using a graph tool
see the attached figure

the solution is
x=12.5 g-----> the mass of each chocolate
y=85 g-------> the mass of the box

the answer is
The mass of each chocolate is 12.5 g