Iwant diagram a vertical tower subtends a right angle on the top of 10m high flagstaff. if distance between them is 20m, find the height of the tower

Let us consider a triangle where AB to be the flagstaff of 10 m and BC to be of 20 m as the distance between the tower and the flag.  
Considering ABC and ADE as triangle
 AB = 10 = CE
 BC = 20 = AE 
 Calculating the values of ABC
 tan x = AB/BC
 tan x = 10/20
 10/tan x = 20(1) 
 Calculating the values of ADE
 tan(90-x) = DE/AE
 cot x = DE/20
 DE/cot x = 20 (2) 
 Solving equations 1 & 2
 10/tan x = DE/cot x
 DE = 40
 Hence the total height of the tower is DE+CE = 40+10= 50 m.