体育场内共有 800 名成人和儿童。其中 70% 是成人。一些成人离开了体育场，成人人数与儿童人数的比变成了 5 : 3。有多少名成人离开了体育场？

1. Calculate the initial number of adults and children

Initial number of adults = $70\% \times 800 = \frac{70}{100} \times 800 = 560$
Initial number of children = $800 - 560 = 240$

2. Use the unchanged number of children to find the new number of adults

Since only adults left, the number of children remains unchanged at $240$.
The new ratio of adults to children is $5 : 3$.
$3 \text{ units} = 240$
$1 \text{ unit} = 240 \div 3 = 80$
New number of adults = $5 \text{ units} = 5 \times 80 = 400$

3. Calculate the number of adults who left

Number of adults who left = $560 - 400 = 160$