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3C means 3 times whatever the value of "C". Similary, 5S means 5 times whatever the value of "S" is.
The Bread Back Then Was Mostly 3c
3c+6=18
3c+6-6=18-6
3c=12
3c÷3=12÷3
c=4
3c-7 = -4
2y-3c = -1