An object starting from rest travels

Ist Case :

Here $u=0, \mathrm{~S}_{1}=20 \mathrm{~m}, t_{1}=2 \mathrm{~s}$

Using $S_{1}=u t_{1}+\frac{1}{2} a t_{1}^{2}$, we get

$20=0+\frac{1}{2} a \times 4=2 a$

or $a=10 \mathrm{~ms}^{-2}$

2nd Case :

$\mathrm{S}_{2}=160 \mathrm{~m}, t_{2}=4 \mathrm{~s}$

$\therefore S_{1}+S_{2}=20+160=180 \mathrm{~m}$

$t_{1}+t_{2}=2+4=6 \mathrm{~s}$

$\therefore 180=\frac{1}{2} a \times 36=18 a$

or $a=10 \mathrm{~ms}^{-2}$

Velocity after $7 \mathrm{~s}, v=u+a t=0+10 \times 7=70 \mathrm{~ms}^{-1}$