Learn some techniques for doing mental addition, such as making groups of 10 and 100.
Addition using groups of 10 and 100 is a free educational video by Khan Academy.It helps students in grades 3 practice the following standards 3.NBT.A.2.
1. 3.NBT.A.2 : Fluently add and subtract within 1000 using strategies and algorithms based on place value, properties of operations, and/or the relationship between addition and subtraction. (A range of algorithms may be used.).