I know this is late but I can still help answer this....
So we know that in December all the deserts altogether had 0.89 mm of rainfall. Now the chart that you have seems to represent the rainfall in August, so we would need to use addition.
0.19 + 0.17 + 0.1 + 0.24 = 0.70 mm
The rainfall altogether would be 0.70 mm in August. Now we compare the difference, we would need to subtract 0.89 mm (December) with 0.70 mm (August).
0.89 - 0.70 = 0.19 mm
There the difference between the two would be 0.19 mm.
Hope this helps!
