Every formula you'll meet — the perimeter of a rectangle, the cost of a phone plan, the fare in a taxi — is just an algebraic expression with the letters standing for real things. Substitution is the skill that makes them all work.
Substitute Real Numbers
The perimeter of a rectangle is P = 2(l + w). For a rectangle 5 long and 3 wide, substitute: P = 2(5 + 3) = 2 × 8 = 16. A skate-park costing cost = 5n + 20 for n visits? Eight visits gives 5 × 8 + 20 = 60 dollars. Same two moves as always — drop the numbers in, then evaluate in order.
Working Backwards
Sometimes you know the answer and need the input. "I spent $45 at the skate-park — how many visits?" Start from 5n + 20 = 45, undo the +20 to get 5n = 25, then undo the ×5 to get n = 5. In the toy's reverse challenge, you slide the input until the formula lands exactly on the target — that's the same idea by feel. Undoing a formula like this is the doorway to solving equations next.
Why This Is the Finish Line
You started with a letter as a placeholder, learned to substitute and evaluate, dodged the 2x-versus-x² traps — and now you can run real formulas both forwards and backwards. That's the move that shows up costing a job, measuring a yard, or sitting an exam. That's mastery.