  hey beautiful- let's shed some light Ignoring the adding of the 1 it becomes more obvious, multiplying by 80 and 250 is same as multiplying by 20,000 which moves the first three digits to the left 4 positions to the left, and doubled. Adding the last four digit twice leaves them in same position but doubled. Dividing then by two you end up with your number. The adding of the 1 just adds 250 to the result, and thus you have to subtract it out later before the divide by 2. ......love, Uncle Chuck 
