Questions/instructions: 

Whakaaturia te käri tangohanga ki te äkonga.

conventional subtraction algorithm; try to take 9 from 5, won’t work ;
convert one ten in top line to units; take 9 from 15, get 6;
take 1 ten away from 2 remaining tens; get 10, total is therefore 16

7

vertical subtraction layout; try to take 9 from 5, won’t work;
add 10 units to 5 in top line; compensate by adding one ten to bottom line;
subtract 9 from 15 (units), get 6; subtract 2 from 3, answer is 16

11

increase 19 to 20; compensate by increasing 35 to 36;
subtract 20 from 36, answer is 16

2

increase 19 to 20; subtract 20 from 35, get 15; add one, answer is 16

5

start at 19 and count up to 35

9

plan to use concrete materials; start with 35, take away 19

2

other

16

take 5 from one container; take 19
(10 from one container, 9 from another);
leaves one with 10, one with 1, and one with 5;
either move 1 to join 5, then add 10 and 6
OR leave separate and add 10, 5, and 1

take 5 beans away/tip all beans out (either order); count 19 beans
and remove them; count the remaining beans

tip all beans out; count 35 from scratch; discard remaining 5; count 19 beans and remove them; count the remaining beans

take 5 beans from container, discard then take 19 beans from containers in
haphazard fashion (no complete 10 left); count beans remaining

take 5 beans from one container, discard; take away two complete containers;
add 1 bean to 5 container from discard pile or discarded containers

other

Commentary:
Just over half of the students were able to explain a valid strategy for question 1. Eighteen percent described a vertical algorithm, seven percent described a part-whole mental strategy. In question 2, 42 percent of students modelled a counting strategy with the materials to complete the subtraction, seven percent modelled a valid part-whole strategy.