There are 500 story books in the school library, 168 are lent to the third grade and 175 are lent to the fourth grade. How many story books are there in the school library?
学校图书室有500本故事书，借给三年级168本，借给四年级175本．学校图书室还有多少本故事书？
<gadget id="calculator">500 - 168 - 175</gadget>
<output>157</output>

<result>157</result>