6. A rectangular chocolate bar is made of equal squares. Irena breaks off two complete strips of squares and eats the 12 squares she obtains. Later, Jack breaks off one complete strip of squares from the same bar and eats the 9 squares he obtains. How many squares of chocolate are left in the bar?
A 72
В 63
C 54
D 45
E 36
SolUTion
D

Since Irena breaks off two strips containing 12 squares of chocolate, one strip contains six squares. Since Jack breaks off one strip containing nine squares, he must have broken his strip from a longer side of the bar while Irena broke her strips from a shorter side of the bar. Therefore the remaining bar is nine squares long and five squares wide and hence now contains 45 squares of chocolate, as illustrated in the diagram below.