Well if you use the equation $500=$125+20x, where x=the number of weeks it took to cover the cost, you have to subtract $125 from both sides and you then get 375=20x. Then divide each side by 20, and you get 18.75=x and then round the answer to 19.
So your answer would be 18.75 or 19 weeks (depending on which answer the problem is looking for).
We know that 50% of the number is 150 so we can assume the number is 300(if its 50% just double that number). Next we use the equation 0.80 x 300 which gives us the answer 240.
