In grade 4, after students gain proficiency with the multiplication facts, they move on to single X double digit multiplication.
The key understanding for this is the distributive property. Having explored this for the basic X facts, they just need to split the double digit number on the decades and use their basic facts.
For example, 4 X 95 is 4 X 90 and 4 X 5.
This is difficult for many students to understand abstractly as written above – it must be presented visually for these students. Arrays are suggested to be the most suitable model, especially when they have been used for the multiplication facts and they can just be extended. For this reason, four different Ontario curriculum expectations suggest the use of arrays in grade 4 multiplication.
Another underlying skill is understanding how to apply their basic facts to something like 4 X 90. Before moving to a generalized rule they should understand the idea of 4 groups of 9 tens, so 36 groups of ten, so 360. This can be explored with mini ten frames.