A book has 150 pages in total. Xiao Ming reads 21 pages every day. He has read it for 5 days. How many pages are left?
一本书一共150页，小明每天看21页，已经看了5天，还剩几页没看？
<gadget id="calculator">21 * 5</gadget>
<output>105</output>

<gadget id="calculator">150 - 105</gadget>
<output>45</output>

<result>45</result>