40 is 4 tens. So 6 × 40 is 6 × 4 = 24 tens. 24 tens is 240.
Worked example
3 × 10 = ?
Answer:30
10 is 1 tens. So 3 × 10 is 3 × 1 = 3 tens, which is 30.
For grown-ups: Multiply one-digit by multiples of 10. Below: three problems to try with the worked answers, and a free printable sheet. Children can practise the same skill as a game, with no sign-up and no ads.
CCSS 3.NBT.A.3
What this practises, for teachers
Practices multiply one-digit by multiples of 10 (3.NBT.A.3). A printable sheet with its answer key, worked answers on the page, no accounts.