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When a number is increased by 20% it is multiplied by 100% + 20% or 120% or 1.2
When a number is decreased by 50% it is multiplied by 100% - 50% or 50% or 0.5
So when a number is multiplied by 1.2 and then multiplied by 0.5 it is
multiplied by 1.2×0.5 = 0.6 which is 60%
Edwin