13+(3+7)=(13+3)+7 what property is that

Need this asap if anyone can i would be very grateful. if you could show workings that would really

Using only the digits 5, 6, 7, 8, how many different three digit numbers can be formed if no digit is repeated in a number?

Lana's family entered a 5-kilometer race lana's average step length is about 0.5 meter how many steps will she need to take the finish