Power the Reactor to Exactly 31 — Grade 2 Two-Step Word Problems

The reactor needs exactly thirty-one units and the cells on the rack don't come labeled with the answer. Finding the combination that lands on the target is the shape 2.OA.A.1 actually asks for — an unknown that isn't sitting on its own at the end of an equals sign. Second graders who are fluent at 17 + 14 can still be stopped cold by "what plus 17 gives 31", and that gap is worth finding early. Five targets per run, all within 100.

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What this station asks for

⚛️ Restart the Reactor CoreWe're docked, but the lights are out. Tap the power cells that add up to exactly the target and slot them into the core. Too much power overloads it, so balance it just right! You pick the power cells that add up to the exact target and restart the core. Five tasks, then a snack wrapper goes into the Case File.

Five questions it might ask

Drawn from the same pool the activity uses, so these are the real thing rather than an illustration. The numbers change on every attempt — there is no fixed set to memorize.

Question	Answer
Power the reactor to exactly 22	22
Power the reactor to exactly 29	29
Power the reactor to exactly 37	37
Power the reactor to exactly 18	18
Power the reactor to exactly 36	36

👀 What to watch for

Trial and error is a legitimate starting point here, not a failure. The progression you're looking for is from picking cells at random, to picking one sensible cell and working out the partner, to reading the target and knowing the pair. All three look the same from across the room.

The Common Core standard behind it

Official wording, quoted in full — not our summary of it:

2.OA.A.1	Use addition and subtraction within 100 to solve one- and two-step word problems involving situations of adding to, taking from, putting together, taking apart, and comparing, with unknowns in all positions, e.g., by using drawings and equations with a symbol for the unknown number to represent the problem.

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Other Grade 2 skills in this case

Addition within 1002.NBT.B.5 Subtraction within 1002.NBT.B.5

All grades

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