Every door shows you the result and hides the starting number, work backwards with inverse operations and crack it before the charge runs out.
Each door displays an equation with the answer already given and the input missing. Reverse the operations, undo a plus with a minus, a times with a divide, to recover the value that makes it true, then key it in. Every door starts with a full charge that ticks down, so solve and enter before it empties. Clear the run of doors to finish.
Boost Code drills inverse operations by flipping the usual problem, you are handed the answer and must rebuild the steps that reach it.
An equation with the answer given and the input missing is solved by inverting it — undo a plus with a minus, a times with a divide — which is the idea of the inverse function at its most concrete.
Working backwards through a chain means applying the inverses in reverse order, which is the part that trips people.
That reversal is not a quirk of the puzzle but a property of composition: to undo "double it, then add three" you subtract three and then halve, not the other way round. The game makes you feel the order because the door will not open otherwise.
Boost Code is an original, free browser math game that drills inverse operations by flipping the usual problem, you're handed the answer and must rebuild the question. No sign-up needed.