The store stipulates that 4 empty soda bottles can be exchanged for a bottle of soda. There are 28 students in a class going on a spring trip. How many bottles of soda should they buy at least to ensure that everyone has a bottle of soda to drink?
商店规定4个空汽水瓶可换一瓶汽水，某班28位同学春游，他们至少买多少瓶汽水才能确保每人有一瓶汽水喝？
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 + (1/3)</gadget>
<output>4/3 = around 1.333333</output>

<gadget id="calculator">28 / (4/3)</gadget>
<output>21</output>

<result>21</result>