Find the area of a rectangle if its legs are 4 cm and 5 cm.

Since in the proposed task we are talking about legs, then with a high degree of probability we can assume that we are talking not about a rectangle, but about a right-angled triangle.
Recall that the area of a right-angled triangle is easier to calculate than the area of an arbitrary triangle:
S = (1/2) * a * b,
where S is the area of a right-angled triangle,
a and b – legs of a right triangle.
Let’s use this formula:
S = (1/2) * a * b = (1/2) * 4 * 5 = 20/2 = 10 (cm 2).
Answer: 10 cm 2.