We know that:
6 < 3r-45 < 36
We want to re arrange the inequations:
Step 1: all terms with an unkown value on one side.
We want to let 3r "be alone" in the middle side:
6 < 3r-45 < 36
6 + 45 < 3r < 36 + 45 [adding 45 in each part]
51 < 3r < 81 [6 + 45 = 51 and 36 + 45 = 81]
Step 2: we want to clear r
51 < 3r < 81
51/3 < r < 81/3 [dividing by 3 all sides]
17 < r < 27 [51/3 = 17 and 81 /3 = 27]
