11. Usain runs twice as fast as his mum. His mum runs five times as fast as his pet tortoise, Turbo. They all set off together for a run down the same straight path. When Usain has run $100 \mathrm{~m}$, how far apart are his mum and Turbo the tortoise?
A $5 \mathrm{~m}$
B $10 \mathrm{~m}$
C $40 \mathrm{~m}$
D $50 \mathrm{~m}$
E $55 \mathrm{~m}$
Solution: $\mathbf{C}$
When Usain has run 100m his mum has run half this distance, that is, $50 \mathrm{~m}$ and Turbo has run onefifth of his mum's distance, that is, $10 \mathrm{~m}$. So the distance between his mum and Turbo is $(50-10) \mathrm{m}=40 \mathrm{~m}$.