What This Drill Trains
Finding a missing addend is algebraic thinking without the symbols. Problems like 9 + ? = 15 force you to use inverse operations — the same habit you need for equations, balance problems, and checking your own work. This drill strengthens number relationships rather than only left-to-right calculation.
How to Think During the Round
Rewrite the problem as subtraction: if a + ? = c, then ? = c − a. Prefer complements to ten when the total is near a round number. Stay flexible: sometimes counting up from the known addend to the total is clearer than formal subtraction.
You have 60 seconds. Accuracy still outranks raw speed: one calm method repeated well beats a streak of guesses. After the timer ends, notice which problems felt sticky — that short review is where real improvement happens.
Worked Examples
Study these before you press Start. Try to recreate the thinking without looking at the answer first.
8 + ? = 15
Thinking: 15 − 8 = 7. Or count up: 8 → 15 takes 7.
Answer: 7
? + 6 = 14
Thinking: 14 − 6 = 8. Same idea even when the blank is first.
Answer: 8
9 + ? = 17
Thinking: 9 + 1 = 10; need 7 more to reach 17 → 8.
Answer: 8
Common Mistakes
- Adding the two visible numbers instead of finding the gap.
- Subtracting in the wrong direction (a − c instead of c − a).
- Ignoring the position of the blank and answering as if the equation were fixed left-to-right.
Practice Tips
- Always name which number is the total before you calculate.
- If you miss several, switch to untimed paper practice with the blank in different places.
- Pair this drill with Single-Digit Sprint — addition fluency feeds missing-number speed.
Suggested Session Plan
- Read the method section once if this skill is new.
- Cover the answers and re-solve the three worked examples aloud.
- Start the timed round and stick to one strategy.
- Replay once, then write down any fact or pattern you missed.
- If the same error repeats, open the full lesson for more examples and a practice set with an answer key.
Full explanation, extra worked examples, and practice problems for this skill: open the related stage lesson .
Prefer a structured curriculum instead of random drills? Browse all learning stages or read the complete mental arithmetic guide . More timed drills live on the games hub.