A ladder 10 m long is leaning against a vertical wall. It makes an angle of 60° with the ground. How far is the foot of the ladder from the wall?

Let say,height of the wall is 'a' and

distance between the foot of the wall and foot of the ladder is 'b'.

Given that ,height of the ladder is 10 m.

So,$$a^2+b^2=10^2$$

or,$$a^2+b^2=100$$..........(1)

And angle between the ladder and ground is 60°.

so, $$tan\theta=tan60°=√3.$$

Or,$$√3=(a/b).$$

or,$$a=√3b.$$

put this value in equation (1):

$$(√3b)^2+b^2=100$$

or,$$4b^2=100$$

or,$$b^2=25$$

or,b=5.(-5 can not be the value of distance)

So,a=5√3.

So,the ladder is 5 m away from the base of the wall.

A is correct choice.