multiply by 8 on both sides
(20)8 - 8(4b/8) = (18)8
160 - 4b = 144
subtract 160 on both sides
160 - 160 - 4b = 144 - 160
-4b = -16
divide by -4 on both sides
-4b/-4 = -16/-4
b = 4
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