16. Chloe chose a three-digit integer with all its digits different and wrote it on lots of pieces of paper. Peter picked some of the pieces of paper and added the three-digit integers on them. His answer was 2331. How many pieces of paper did Peter pick?
A 2331
B 21
C 9
D 7
E 3
16. C First note that the factorisation of 2331 into prime numbers is $2331=3 \times 3 \times 7 \times 37$. Note also that $3 \times 37=111$. Since Claire's three-digit integer has all its digits different, and both $3 \times 111=333$ and $7 \times 111=777$ have repeated digits, the factors of Claire's integer do not include both 3 and 37 . However, since $3 \times 3 \times 7=63$, which is not a three-digit integer, the factors of Claire's integer must include 37. Therefore Claire's integer is $37 \times 7=259$ and hence the number of pieces of paper Peter picked is $2331 \div 259=9$.