Student Weight Distribution: Median and Underweight Percentage

During a health check-up, the weights of \(20\) students of the same height were recorded.

The stem-and-leaf diagram below illustrates their results.

1. If the median weight of the students is \(56\) kg, write down the value of \(w\).
2. A student is considered underweight when weighs less than \(45\) kg. Find the percentage of students who are underweight.