The binary number formed by prefix `[0..i]` can be computed incrementally: `num = (num * 2 + nums[i]) % 5`. Because modular arithmetic is idempotent under multiplication, taking mod at each step yields the same result.

Track the current prefix value modulo 5. At each step, update it as `(curr * 2 + bit) % 5` and check if it equals 0.

The key insight is that `(a * 2 + b) % 5 == ((a % 5) * 2 + b) % 5`, so we only need to keep the remainder.