Let A(x) denote the number (in hundreds) of computers sold when x thousand dollars is spent on advertising. Represent the following statement by equations involving A or Upper A prime: When three thousand dollars were spent on advertising the number of computers sold was 1800 and it was rising at the rate of 80 computers for each 1000 dollars spent on advertising. (b) Estimate the number of computers that will be sold if $4000 is spent on advertising.