There are areas of math where problems have variables that do change. But it's clear from the problem statement which variables change and which don't. E.g. a diver steps off the diving platform that is 5 meters high. If the initial speed is 0 m/sec what will be his speed when he touches the water? Here the speed is 0 m/sec initially, but we know from observing falling apples that it increases based on certain laws of physics. But the platform height stays the same - we can assume that while the diver is in the air nobody drained the pool.