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

when the sides are congruent (same direction and size), and the two opposite sides are parallel, the angles are the same. in an isosceles triangle, two angles are congruent, because two sides are congruent.