Percenatages are based on 100% so if you have a 5% increase you will take your amount times 1 + 0.05, which is 1.05 or 105%.
This is the same as taking your amount times 5% then adding the two amounts together.
Take your problem for example if your sales were 8,000 and you have a 50% increase, your increase will be 4,000 units. 8,000 units + 4,000 units will be 12,000 units. But if you take 8,000 units times 150% or 1.05 you will come up with 12,000 units, which is a one step process, versus a 2 step process the other way.
You can use either method, which ever is the easiest for you to use.
If you have a decrease in percentage you will subtract the amount instead of adding the percentage. For an example you expect sales to decrease 5% you will take your actual sales for one year times 95%, which is 1 - 0.05 or 0.95 to get the estimated amount in one step or you could multiply your actual sales by the decrease in percentage then subtract this number from the actual sales to get your estimated sales, which will be a 2 step process.
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