18. Peri the winkle leaves on Monday to go and visit Granny, $90 \mathrm{~m}$ away. Except for rest days, Peri travels $1 \mathrm{~m}$ each day (24-hour period) at a constant rate and without pause. However, Peri stops for a 24-hour rest every tenth day, that is, after nine days travelling. On which day does Peri arrive at Granny's?
A Sunday
B Monday
C Tuesday
D Wednesday
E Thursday
Solution: $\mathbf{C}$
Until Peri reahes Granny's, he travels $9 \mathrm{~m}$ in 9 days and then rests for a day. So he travels $9 \mathrm{~m}$ in 10 days. So it takes him 90 days to travel the first $9 \times 9=81 \mathrm{~m}$. Then after a further 9 days he has travelled the final $9 \mathrm{~m}$ and has reached Granny's. So the journey takes him 99 days. Now $99=14 \times 7+1$, so the journey takes him 14 weeks and 1 day. Therefore he arrives on a Tuesday.