Answer:
250 mm^2
Explanation:
There are four separate rectangular areas here. If you look closely, you'll see that each such subarea is 5 units wide. The lengths (or heights, if you wish) are {20, 15, 10, 5}.
Area = (base)(height). We can find the area of the first (longest) subarea by multiplying (20)(5), obtaining 100.
Just multiply the set {20, 15, 10, 5} by 5, obtaining the set {100, 75, 50, 25}.
Now add up these four numbers (which are the areas of subareas), obtaining:
175 + 75 units^2, or 250 mm^2.