Free printable math drill — download and print instantly
This Addition Within 20 drill has 40 problems for Grade 2. Detectives theme. Answer key included.
⬇ Download Free Math DrillGet new free worksheets every week.
All worksheets checked by our AI verification system. No wrong answers — guaranteed.
Max found mysterious clues scattered across the detective agency! He must solve addition mysteries before the criminal escapes town.
Standard: CCSS.MATH.2.OA.B.2
Addition within 20 is a cornerstone skill for second graders because it bridges the concrete counting strategies they used in first grade with the mental math fluency they'll need for multiplication and two-digit problems later. At ages 7-8, children's brains are developing the ability to hold multiple numbers in mind simultaneously and recognize patterns—like knowing that 7 + 5 is the same as 5 + 7. Mastering these combinations builds automaticity, meaning students can recall answers quickly without counting on their fingers each time. This frees up mental energy for solving word problems, understanding money, and tackling real-world scenarios like sharing snacks or keeping score in games. When students can add fluently within 20, they gain confidence in math class and develop the number sense that makes all future math more accessible and less intimidating.
Many second graders struggle when they try to count on from 1 instead of from the larger number—for example, adding 3 + 15 by counting 1, 2, 3, 4... all the way to 18, which is slow and error-prone. Others lose track of their count by using fingers inconsistently or forget how many they've already counted. Watch for students who consistently miscalculate sums near 10 (like 6 + 5 or 7 + 4), which signals they haven't yet mastered the "making 10" strategy that makes these combinations easier. You'll spot these patterns when a child counts aloud slowly, restarts frequently, or gives different answers to the same problem on different attempts.
Create a simple "number detective" game at home using two dice or number cards (1-10). Have your child roll or draw two numbers, then find the sum together—but first, ask them to predict whether the answer will be more or less than 10. This prediction step activates their number sense before computing. Play for just 5-10 minutes a few times a week during meals or car rides. Celebrate when they start recognizing shortcuts, like "I know 9 + 9 is 18 because it's almost 10 + 10."