Explanation :
$V_A=r_{A\cdot w}=80\;m/s$
$V_B=r_{b/cdot w}=140\;m/s$
$\frac{r_b}{v_b}=\frac{r_A}{v_A}\;\;\;\therefore r_b=\left(\frac{v_B}{v_A}\right)\cdot r_A=\left(\frac{140}{80}\right)\cdot r_A=1.75\;r_A$
we have given $r_b-r_A=300$
$1.75\;r_A-r_A=300$
$r_A=300/0.75=400$
so,$r_b=300+r_A=300+400+700\;mm$
$\therefore$ Dia. of wheel is 1400 mm .