在一次聚会上，20% 的人是男士，其余人中 55% 是女士，剩下的是儿童。当时有 100 名男士。聚会上有多少名儿童？

1. Find the total number of people at the party

$20\%$ of the people $= 100$
Total number of people $= \frac{100}{20} \times 100 = 500$

2. Find the number of remaining people

Remaining people $= 500 - 100 = 400$

3. Find the percentage of children out of the remaining people

Percentage of children among the remaining $= 100\% - 55\% = 45\%$

4. Calculate the number of children

Number of children $= \frac{45}{100} \times 400 = 180$