Percentages

Water:
Sand:
Start Value - 150 mL
End Value - 350
Difference - 200
200/150 = 1.33*100 = 133.33%

Soil:
Start Value - 150 mL
End Value - 440 mL
Difference - 290
290/150 = 1.93*100 = 193.33%

The sand is more prone to sliding because it takes less water for it to start sliding.