4. A ski resort has an upward-moving escalator. Brother Bear and Little Bear are in a hurry to get up the mountain. While the escalator is moving them up, they are also walking up. Brother Bear took 15 minutes to get from the foot to the top of the mountain, while Little Bear took 30 minutes. One day, the escalator was out of service and stopped running. Brother Bear took 20 minutes to walk from the foot to the top of the mountain, while Little Bear would need $\qquad$ minutes to walk from the foot to the top of the mountain.
$60$