Every homework, quiz and test goes into the machine. Out comes your grade. Pull the levers and see what changes it.
Your grade is not your last score. It is all your scores mixed together. One great test helps, and one rough test hurts, but neither one decides everything.
With weighted grades, tests often count for half. With total points, a 100-point test counts ten times more than a 10-point homework. Either way, big things move your grade more.
A 70% still earns 70% of the points. Missing work earns nothing at all. Turning in something, even if it is not perfect, almost always beats turning in nothing.