# 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?

##### 1 Answer

#### Explanation:

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