A special school is a school catering students with special educational needs, for
example, students with an intellectual or physical disability. Similar to students at regular
schools, students at special schools in Indonesia also learn the basic mathematics needed to
improve their ability to live independently. For example, the curriculum guidelines at
special schools for the intellectually disabled advocate the teaching of standard written
algorithms during the primary levels, and that the algorithms should be adjusted to take
account of students’ needs and abilities. Research on mental calculation strategies of those
students is lacking even though there is a need to understand how these students calculate
mentally. This paper describes the mental calculation strategies of a student attending a
special school for the intellectually disabled and discusses how the taught standard written
algorithm might affect her mental strategies. The paper concludes with recommendations
for what teachers can do to help such students to enhance their mental calculation
strategies.