The real-world problem most likely to involve irrational numbers is:
Finding the diameter of a circle based on the radius.
This is because the relationship between the diameter (d) and the radius (r) of a circle is given by the formula \( d = 2r \). However, when dealing with circles, often the use of the radius also relates to calculations involving π (pi), which is an irrational number. For instance, if you need to calculate the circumference or area of the circle using the radius, you would also be working with π, leading to irrational results.
In contrast, the other scenarios (finding the width of a rectangle from area and length, side length of a square from area, and calculating total cost) do not necessarily involve irrational numbers, as they can often be computed with rational numbers.