The authors begin with a nice observation. After interviewing a number (unspecified) of 3rd
and 4th grade students, they found none who correctly answered a particular problem in
subtraction of whole numbers, namely “7002-25=?”. They also learned that students who
get answers that are egregiously wrong seem not to be able to recognize that something
has gone wrong. These students do not use such potentially helpful equipment as analogies
with money-changing, Dienes’s MAB blocks, or approximate calculation to do estimates.