Lets D be the distance travelled by Chad when meeting Wei Liang and x be the time Chad and Wei Liang met
730-700=0.5h <=============How much earlier did Chad begin before Wei Liang
1/2*6km/h=3km <====Distance travelled b chad before WL begun.
3km/h+6km/h=9km/h <======Total Speed travelled by WL and C
12-3=9km<=====Distance travelled by Wei Liang and Chad together
9km/9km/h<========Time taken for WL to meet Chad