Hello,

Given that the distance is directly proportional to the square of time.

So, d ∝ t^2

So, d = k.t^2

Where, k is the constant of proportionality.

Now from first condition,

d = 78m and t = 7 sec.

So, 78 = k. 7^2

So, k = 1.59 = 1.6

Now, d = k. t^2

So, for first four seconds, t = 4sec

So, d = k. 4^2

So, d = 1.6 x 16

So, d = 25.6 m

So, The stone will fall a total distance of 25.6 m in 1st 4 seconds.

Best Wishes.