The Common Core State Standards teach students the algorithm for multi-digit addition and subtraction in 4th grade. But some experts say it should be taught sooner.

Tape Diagrams: 2-Digit Addition and Subtraction | Grade 2. Special | 2m 20s. Your child continues to use tape diagrams as a visual tool to solve word problems.

The standard method is based on place value. The 10s digit is representing some number of 10s, and it's possible to move or borrow one of those 10s over to the ones place, if we need to.