Mental Math Within 20: Add and Subtract in Your Head

This Grade 1 mental math worksheet lines up twelve addition and subtraction problems within 20 and asks for no written working at all. The child reads each problem, solves it in their head, and writes just the missing number in the box. Some problems add, some subtract, and the mix keeps every line honest — twelve small victories on one quiet page.

Mental math within 20 is the engine room of elementary arithmetic. These are the facts that column methods, word problems, and eventually multi-digit work all lean on, and the goal is fluency: answers that arrive quickly and calmly, without finger counting. A mixed page like this one keeps the child genuinely reading each problem instead of settling into autopilot, and the missing-number format sneaks in early algebraic thinking about what makes an equation true.

The worksheet prints free as a PDF, so it works beautifully as a warm-up routine — same format, fresh effort, a few minutes a day. Children who hesitate on certain facts can step back to number bonds to 10 to rebuild the pairs underneath, while confident solvers are ready for fact family houses, where the same numbers show off their family ties. A calm minute of mental math each morning adds up faster than any single long session.