Start from zero — the basic math you need before everything else
Mathematical notation can look intimidating at first glance, but it is really just a set of very precise instructions packed into a tiny space. A single small symbol tells you which value to grab, where to start, where to stop, and whether to add things up or multiply them together. Once you learn to unpack one instruction at a time, a formula that used to look like a wall of squiggles turns into a short recipe you can just follow.
Picture the cloakroom at a busy theatre. You hand over your coat and get back a ticket with a number stamped on it, say 14. That ticket says nothing about your coat. It isn't blue, it isn't a raincoat, it just tells the attendant which hook to check. Later they read your ticket, walk straight to hook 14, and hand you back exactly your coat, not anyone else's. Today the cloakroom has five hooks, holding coats that weigh 4, 7, 2, 5, and 3 kilograms, in that order. A small number sitting just below and to the right of a letter does exactly the same job in maths. It tells you which entry in a list to look at, not what that entry is worth.
Try it yourself below. The five hooks and their coats are drawn as bars you can build up one at a time. Drag the slider to choose how many hooks you have checked so far, and watch the running total grow term by term. Flip the switch and the same hooks get multiplied together instead of added.