A boy can throw a stone up to a maximum height of 10m .The maximum horizontal distance that the boy can throw the same stone up to will be

Ans:20

But I calculated it using R/H=4/tan(45)

Since maximum range I took tan45...The answer came out to be 40m ...COuld you please tell me what is wrong with the method I used...

For Vertically projected up body maximum height (H) is given as
H = u^2 / (2g) ……[1]

For the same body with same initial initial velocity the horizontal distance travelled when thrown at an angle (θ) is given as
R = u^2 Sin(2θ) / g
Maximum horizontal distance is given as
R = u^2 / g ……[2]

Compare equations 1 and 2
R = 2 H
R = 2 × 10
R = 20 m