The mole fraction of urea ($M = 60\,\text{g/mol}$) in an aqueous solution containing $6\,\text{g}$ of urea in $180\,\text{g}$ of water is approximately:

Moles of urea $= 6/60 = 0.10$. Moles of water $= 180/18 = 10.0$. Mole fraction of urea $= 0.10/(0.10 + 10.0) = 0.10/10.10 \approx 0.0099 \approx 0.010$.