Question

Which strategy can be used to multiply 6 x 198 mentally?

Answer 1

Start from the right side and say
6 times 8 is 48 carry the four, 6 times 9 is 54 add your 4 you carried that gives you 58, carry your five and say 6 times one is 6 add your carried 5 and you get 1188

Answer 2

198 is the bigger number, so in order to multiply mentally, we multiply 6 to every number in 198.

What I mean is: 
6 x 8 = 48
4 is added to answer of 6 x 9.
6 x 9 = 54 + 4 = 58
5 is added to answer of 6 x 1.
6 x 1 = 6 + 5 = 11.

6 x 198 = 1188