Also, if it takes you, say, 2 minutes to struggle through a simple multiple choice algebra problem that you're able to overcome through lots of brute force methods but you still get the right answer, you can pass algebra tests easily, but you're going to struggle when you hit calculus. You need to be able to solve some of those problems mostly in your head in seconds, and the methods need to be intuitive and familiar to you. Moreover, the rest of your work depends on having the right answer to that problem. If you can get the right answer 90% of the time with a lot of effort that might be good enough to pass a class, but it's not actually good enough to use those skills practically. In an easy calculus problem you might have to work through dozens of little simple algebra problems, and if it takes minutes to do each one, and you get the wrong answer 10% or more of the time, then calculus problems are going to take hours and you're going to get the wrong answer most of the time.