Be a Number Detective Math Center Work First Grade is a creative way for your students to practice addition and subtraction skills during math centers, or whole class time. Just provide a magnifying glass, and let them become number sentence detectives! Included in the 10 pages are: sums, differences, doubles, missing numbers (both addends and subtrahends), and three addends. Each page also has *4 Challenge problems to 25. These printables are meaningful, and the students will definitely be engaged, and will be practicing needed first grade skills in an enjoyable way!