The price of the homework books sold by the three stationery stores is 0.5 yuan each, but the three stores have different promotions: Pippi Stationery Store: 10% off for all. Lele Stationery Store: Buy 5 get 1 free. Its stationery store: 20% discount for over 55 yuan. Now Teacher Wang needs to buy 100 homework books. Which stationery store is more cost-effective? How much is the minimum cost?
三家文具店出售的作业本的价格都是每本0.5元，但是三家店都有不同的促销活动：皮皮文具店：一律打九折．乐乐文具店：买5送1．其其文具店：满55元八折优惠．现在王老师需要构买100本作业本，选择哪家文具店比较合算？最少要花多少元钱？
<gadget id="calculator">80 + 4</gadget>
<output>84</output>

<gadget id="calculator">0.5 * 84</gadget>
<output>42</output>

<result>42</result>