distance travelled in first 10s is -

 ${\mathrm{X}}_1=0+\frac{1}{2}\times \mathrm{a}{\left({\mathrm{t}}_1\right)}^2$ 

= $\frac{1}{2}$ a. (10)² = 50 a        .... (i)      

Velocity of particle at point B is

 V = 0 + a .10 = 10a         .... (ii)

Then distance travelled between B & C i.e. X₂ in next  10 second.

X₂ = V × 10 + $\frac{1}{2}$ a (10)² 

= 10a × 10 + $\frac{1}{2}$  a × 100 = 150 a    .... (iii)

From equation (i) & (iii), $\frac{{\mathrm{X}}_2}{{\mathrm{X}}_1}=\frac{150\mathrm{a}}{50\mathrm{a}}=3$ 

X₂ = 3x₁