Problem 4 · The smallest with digit sum 15
Minimising left to right, pushing the weight to the end.
Integer answer, at most 4 digitsWhat is the smallest four-digit number whose digits add up to 15?
Copa Cangur · SCM
Easy
Closed answer
Reasoned solution
Key idea: to minimise, make the leading digits as small as possible and leave the sum for the last ones.
First digit: $1$ (smallest possible). Second: $0$. That leaves $14$ for two digits: a digit is at most $9$, so the last digit is $9$ and the third is $14 - 9 = 5$.
$$1059: \quad 1 + 0 + 5 + 9 = 15.\ \checkmark$$
Answer: 1059