Aright triangle abc is shown below:

a right triangle abc with height ab labeled 4 units and base bc labeled 5 units is shown.

the area of the triangle above will equal one half of a rectangle that is 5 units long and __ units wide. (input only whole numbers, such as 8.)

The answer is:  "4" .

Note:  The question asks for a whole number.

"The area of the triangle above will equal one half of a rectangle that is 5 units long and   4   units wide.  (Input only whole numbers, such as 8.).

Note:  The diagram shows a "right triangle" ; which would be "one-half" of a rectangle, with units of "length" being "5 units" (as mentioned in this very question prompt;  and a width of "4" units (as shown in the diagram;  hence the answer is "4" units). 

Furthermore, the formula/equation for the area, "A" , of a triangle is: 

A = (1/2) * b * h ; 

that is: 

Area = (1/2) * (base length) * (perpendicular height).

As such, the answer, "4" (units) makes sense.