6. The length of a set of four well-parked and fitted trolleys is $108 \mathrm{~cm}$. The length of a set of ten wellparked and fitted trolleys is $168 \mathrm{~cm}$.
What is the length of a single trolley?
A $60 \mathrm{~cm}$
B $68 \mathrm{~cm}$
C $78 \mathrm{~cm}$
D $88 \mathrm{~cm}$
E $90 \mathrm{~cm}$
SOLUTION
$\mathbf{C}$

Six extra trolleys added $60 \mathrm{~cm}$ to the length of the set. Therefore, each extra trolley adds $60 \mathrm{~cm} \div 6=10 \mathrm{~cm}$ to the set. Since a set of four trolleys has length $108 \mathrm{~cm}$, the length of a single trolley is $(108-3 \times 10) \mathrm{cm}$, or $78 \mathrm{~cm}$.