and $ L $ is a multiple of 7 (since pipes are seven units long and total length is a sum of such pipes). However, note: the problem states the *total length of installed pipe* is one more than a multiple of 13—so $ L \equiv 1 \pmod13 $, and $ L $ must be expressible as a sum of multiples of 7. But since individual pipes are 7 units, any valid total length must be a multiple of 7. So we seek the smallest two-digit number $ L $ such that: - go-checkin.com