Hey,

since the speed is increased twice therefore initial speed is 120/2=60 km/hr.

now, there are two cases:

Case I:

given,

u=60 km/hr

v=0

S=20 m

Applying formula: v^2=u^2+2aS

0=(60*5/18)^2-(2*a*S)

a=(60*5/18)^2/(2*20)

Case 2:

u=120 km/hr

v=0

a=(60*5/18)^2/(2*20) (from case 1)

Applying the same formula as case 1

v^2=u^2+2aS

0=(120*5/18)^2+2*(60*5/18)/(2*20*S)

therefore

S=80 m, which is the required distance and answer.

I hope this helps.

All the best!