Arthur J. Baroody, The Development of Counting Strategies for Single-Digit Addition, Journal for Research in Mathematics Education, Vol. 18, No. 2 (Mar., 1987), pp. 141-157 ...

Place value in two-digit numbers When you are solving an addition calculation with a two-digit number, you need to think about tens and ones.

The present paper describes the success, the methods (mental, informal written, standard algorithm) and the strategies of informal written arithmetic to be observed when 300 elementary students worked ...