$500
x*.8=400
divide .8 from both sides
x=500
40 x ( $84 / 8 ) = ( 40 / 8) x $84 = 5 x $84 = $420
first day is 5; second day is the square of 5, so it's 25; third day is the 3 times of the second day, so it's 25x3, which is 75. Add these 3 days together to get the answer, which is 105 dollars.