There were 800 tourists in the Science and Technology Park in the morning, and 300 people left at noon. Another 400 tourists came in the afternoon. How many tourists were there in the park at this time?
科技园上午有游客800人，中午有300人离去．下午又来了400位游客，这时园内有多少游客？
<gadget id="calculator">800 - 300</gadget>
<output>500</output>

<gadget id="calculator">500 + 400</gadget>
<output>900</output>

<result>900</result>