Math Strategy

I have learned how to work out my 6 x,7 x and 8 x from my 5 x table, I will show you what I mean:
3 x 6 = 18
(3 x 5)= 15 + (3 x 1)= 3
15 + 3= 18

I will show you another time table:
3 x 7=21
(3 x 5)=15 + (3 x 2)=6
15 + 6=21