Which statement shows the associative property of addition? HELP

3⋅(5⋅x)=(3⋅5)⋅x

21 + 2y = 2y + 21

If m + 3 = 18, then 18 = m + 3.

(u + 7) + 13 = u + (7 + 13)
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At the newest animated movie, for every 9 children, there are 4 adults. there are a total of 39 children and adults at the movie.

Acarnival sold tickets for $1.50 for adults and $1.00 for students. there were 54 tickets sold for a total of $70.50. write a system of equations to represent the number of adult tickets,x, and the number of student tickets,y. find the solution and explain what it means. i'll you if you me

Kapil needed to buy a long wooden beam. he went to two sawmills that each charge an initial fee plus an additional fee for each meter of wood. the following equation gives the price (in dollars) of a wooden beam from the first sawmill as a function of its length (in meters). p = 5+20xp=5+20x