Hello again Andrea. When answering a problem like this 1st get an idea in your head what a reasonable answer would be. When I see that he texted 36 times in 2 1/4 hours I can estimate pretty quickly that he is texting less than half that amount in only one hour. That eliminates c and d immediately. Now to get the exact answer I need to set up a proportion but when I do that I have to make sure that I have the same things in both numerators and the other thing in both denominators. I am going to choose to put number of text messages in the numerators(one is 36 and the other is unknown and we will call "x". I am going to put time in each denominator(one is 2 1/4 hours and the other is 1 hour because they asked for a unit rate). So now I have two fractions that are equivalent(36/2.25 and x/1). I cross multiply and solve for my unknown "x". x times 2.25 = 36 times 1. x = 36/2.25 = 16. I could have used 9/4 like you wanted. Then it would have been 36/(9/4) = x/1. Then cross multiplying I would get x times 9/4 = 36 times 1. Now you would solve by multiplying each side by the reciprocal (4/9).
(9/4)(4/9)x = (36)(4/9) yielding x = 16. Hope this helps. If you need any further explanation let me know.
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