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Its 75 because you at the 2 variables and then subtract them by 180 so 180-105=75
30,00 watts.
Think of the metric system like this: K H D B D C M
Now. you have kilo at the begining and milli at the end. To transition from left to right you multiply by 10 each time. To transition from right to left, you divide by 10 each time.