y 1=(x﹣4)×5+20×4=5x+60,y2=(5x+20×4)×0.9=4.5x+72.
(2) let y 1=y2, ∴ when x=24, the cost of purchasing by two preferential ways is as much;
(3) Because we need to buy 4 schoolbags and 12 water-based pens, and 12 < 24,
Purchase Scheme 1: Preferential method ① Purchase requires 5x+60 = 5x12+60 =120 yuan;
Purchase scheme 2: two purchase methods are adopted, and the preferential method ① is to buy 4 schoolbags.
Need 4×20=80 yuan, and get 4 water-based pens at the same time;
Using the preferential method ② to buy 8 water-based pens, it takes 8×5×90%=36 yuan.
* * * Cost 80+36= 1 16 yuan. Obviously, 1 16 < 120.