Mathematically, seven-year-olds have strong number sense and estimation skills. Children this age can do simple addition and subtraction and can apply strategies necessary to solve related word problems. They can also effectively work in many ways with three-digit numbers and have improved abilities for solving arithmetic problems mentally. Seven-year-olds use rulers to measure units. They also understand how to measure angles and can apply their knowledge of shapes to three-dimensional objects and structures in the environment.