Math oral p42 answer in the third grade of primary school 1x2+8x2=
* * * multipliers are the same, so they can be combined, 2x( 1+8)=2x9= 18.
When the number is large, consider the number 2 as a thing, such as an apple, and the formula is 1 apple +8 apples. You will naturally know that if you combine them into 9 apples, you can get 18 by substituting apples =2.