Answer: there were 345 pieces of chocolate in the box.
Step-by-step explanation:
Let x represent the number of pieces of chocolate there in the box.
0.55 of a box of chocolate are dark chocolate. This means that the number of dark chocolate in the box is 0.55x
0.3 are milk chocolate. This means that the number of milk chocolate in the box is 0.3x
The rest are white chocolate. This means that the number of white chocolate in the box is x(1 - 0.55 - 0.3) = 0.15x
There are 138 pieces more dark chocolate than white chocolate. This means that
0.55x - 0.15x = 138
0.4x = 138
x = 138/0.4
x = 345
