Suppose the speed of her bike is x kilometers, and do the equation:

0.5*X=(X+4)*(0.5-6/60)

Solution: 0.5*X=(X+4)*(0.5-0. 1)

0.5*X=(X+4)*0.4

0.5*X=0.4*X+ 1.6

0. 1*X= 1.6

X= 16

So her riding speed is16 km/h.