Question

(18×2)×5=18×(2×5) which property they used

Answer 1

Associative Property.

Where  (a *b)*c = a*(b*c) 

Check:
(18×2)×5= 36×5 = 180

8×(2×5) = 18 ×10 = 180

Both answers are the same.

Answer 2

Associative Property is used in both the answers are the same they are just in different orders.