How many feet above the ground does the ladder touch the wall?

A 15-foot ladder is leaning against a 30-foot wall. The bottom end of the ladder is 9 feet from the wall. How many feet above the ground does the ladder touch the wall?

A) 1.7
B) 6
C) 12
D) 17.5
E) 144

What knowledge would one need to work this out?

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia…).