What property is 13+(7+29)=(13+7)+29

Which of these statements is correct? the system of linear equations 6x - 5y = 8 and 12x - 10y = 16 has no solution. the system of linear equations 7x + 2y = 6 and 14x + 4y = 16 has an infinite number of solutions. the system of linear equations 8x - 3y = 10 and 16x - 6y = 22 has no solution. the system of linear equations 9x + 6y = 14 and 18x + 12y = 26 has an infinite number of solutions.

What is number 31 as a sum of tens and ones?

Grace paid $1.70 for 5 ounces of candy. how much did she pay for 24 ounces?