1. The expression for c in the first equation is (30m+50). Substituting that for c in the equation ...
c = 20m +100
gives you ...
30m +50 = 20m +100
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2. Adding -50-20m to both sides gives ...
10m = 50
m = 5 . . . . . . . divide by 10
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3. Doing the substitution in reverse, you would substitute (20m+100) for c in the equation ...
c = 30m +50
to get ...
20m +100 = 30m +50
This is the equation of part 1 with the expressions swapped to the other side of the equal sign. The symmetric property of equality tells you that changing sides of the equal sign does not change the value of the variable(s).
