Dylan is evaluating the expression 13 + 19 + 7 + 10. At one step in his work, Dylan rewrites the equation as 13 + 7 + 19 + 10. Which property of addition must Dylan have used when he evaluated the expression?

What is the missing step in this proof ?

Suppose that in 2008 , 840,800 citizens died of a certain disease assuming the population of teh country is 399 miliion, what is the mortality rate in units of deaths per 100,000 people?

You are standing next to a really big circular lake. you want to measure the diameter of the lake, but you don't want to have to swim across with a measuring tape! you decide to walk around the perimeter of the lake and measure its circumference, and find that it's 400\pi\text{ m}400? m. what is the diameter dd of the lake?