Detailed Instructions: This task involves creating questions from a given passage that involve some kind of complex reasoning (including numerical reasoning).
The generated questions must require looking at more than one part of the passage to answer. Try to use a variety of reasoning types in your questions (some of the sample reasoning types are illustrated in the 'positive examples' field) and also have a variety of answer types (spans, numbers, dates). A span is a continuous phrase taken directly from the passage or question. In such questions, the answer spans are recommended to be five words. Questions with answer type "number", are expected to start with "How many". For instance, "How long did an event last?" can be phrased as "How many years did an event last?". To increase diversity, try to create more questions for which the answer will be multiple spans. Please create questions that require AT LEAST 2 arithmetic operations as illustrated in the 'positive examples' field. You are encouraged to try DIFFERENT COMBINATIONS of operations and not just limit to the combinations shown in those examples. The more challenging questions you create, the better it is.
Q: Passage: In July 1449 Esen Tayisi of the Mongols launched a large-scale, three-pronged invasion of China with his puppet khagan Toqtaq-Buqa. He personally advanced on Datong in August. The eunuch official Wang Zhen, who dominated the Ming court, encouraged the 22-year-old Zhengtong Emperor to lead his own armies into battle against Esen. The size of Esen's army is unknown but a best guess puts it at some 20,000 men. The Ming army of about 500,000 was hastily assembled; its command was made up of 20 experienced generals and a large entourage of high-ranking civil officials, with Wang Zhen acting as field marshal. On August 3 Esen's army crushed a badly supplied Chinese army at Yanghe, just inside the Great Wall. The same day the Emperor appointed his half-brother Zhu Qiyu as regent. The next day he left Beijing for Juyong Pass. The objective was a short, sharp march west to Datong via the Xuanfu garrison, a campaign into the steppe and then a return to Beijing by a southerly route through Yuzhou. Initially the march was mired by heavy rain. At Juyong Pass the civil officials and generals wanted to halt and send the emperor back to Beijing, but their opinions were overruled by Wang Zhen. On August 16, the army came upon the corpse-strewn battlefield of Yanghe. When it reached Datong on August 18, reports from garrison commanders persuaded Wang Zhen that a campaign into the steppe would be too dangerous. The "expedition" was declared to have reached a victorious conclusion and on August 20 the army set out back toward China proper.
A:
Approximately how many more men did the Ming army have, over Esen's army?