In Cuba there are banknotes with values of 3, 10 and 20 pesos. Using only these banknotes, what is the largest amount that can not be formed?
What an intriguing question!
Note that the first 10 multiples of 3 end on every possible digit from 0 to 9.
Arranged so the units are in order from
Every number from
What about the numbers less than 20 that cannot be formed with