Securing mental strategies: calculation up to 999

Spine 1: Number, Addition and Subtraction – Topic 1.19

Primary Year 3 KS2
Mastery PD Materials


Build on segments 1.15 and 1.16 to equip children with useful calculation strategies for bridging hundreds boundaries, and three-digit numbers; continue to use the partitioning structure to facilitate calculation.

Teaching points

  • Teaching point 1: Known partitioning strategies for adding two-digit numbers within 100 can be extended to the mental addition of two-digit numbers that bridge 100, and addition of three-digit numbers.
  • Teaching point 2: Transforming addition calculations into equivalent calculations can support efficient mental strategies.
  • Teaching point 3: Subtraction calculations can be solved using a ‘finding the difference’ strategy; this can be thought of as ‘adding on’ to find a missing part.
  • Teaching point 4: The order of addition and subtraction steps in a multi-step calculation can be chosen or manipulated such as to simplify the arithmetic.