8547 on the board
ask students to multiply this number by any digit
then they multiply this new answer by 13 (without a calculator)
for any digit?
what is special about 8547 I wonder...
let's try to factorise it (using a calculator maybe)
[it is 3 x 7 x 11 x 37 or (7 x 11) x (3 x 37)]
what happens when you multiply 7 x 11 by 13?
and what is 3 x 37?
what happens when you multiply these two bits together?
a similar task might be to multiply 910091 by any digit and then by 11