Find smallest N for 5-and-21 sums

Q: Find smallest N for 5-and-21 sums

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Question

You are given two positive integers: 5 and 21. We say a positive integer x is representable if it can be written in the form

$x = 5a + 21b$

for some non‑negative integers a and b (i.e., using any number of 5s and 21s, order does not matter).

For example, 99 is representable because:

99 = 21 + 21 + 21 + 21 + 5 + 5 + 5.

Find the smallest integer N such that every integer greater than or equal to N is representable.

Output this minimal value of N.

Find smallest N for 5-and-21 sums

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