A ladder 15 m long reaches a window which is 9 m above the ground on one side of street. Keeping its foot at the same point, the ladder is turned to the other side of the street to reach a window 12 m high. What is the width of the street:

Draw a figure as per given question,
In a Right triangle ADC, Use the (Pythagoras Theorem)
AC = √DC² - AD²
⇒ AC = √ 15² - 9 ²
⇒ AC = √ 225 - 81
⇒ AC = √ 144
⇒ AC = 12 cm
In a Right triangle BCE, Use the formula
CB = √ CD² - BE²
⇒ CB = √15 ² - 12²
⇒ CB = √225 - 144
⇒ CB = √ 81
⇒ CB = 9 m
∴ Width of the street (AC + BC) = AB = 12 + 9 = 21 m.