Hellp it's math and I'm bad at math

Compute the standard deviation of the following set of data to the nearest whole number. 10, 10, 15, 20, 25

Will mark brainliest! ( explain the answer too, it can be one sentence)

Jack has a rectangular piece of land, the area of which is represented by a1 = 9.5l. his brother has a different rectangular piece of land, the area of which is represented by a2 = l(14 − l). let a represent the area in square meters and l represent the length in meters of the pieces of land. the two equations plotted on a graph meet at a point as shown in the image