To find the base length of a triangle given its area and height, you can use the formula for the area of a triangle:
\[ \text{Area} = \frac{1}{2} \times \text{base} \times \text{height} \]
In this case, you know the area of the yield sign triangle is 10 sq ft, and the height is 4 ft. Plugging these values into the formula, we have:
\[ 10 , \text{sq ft} = \frac{1}{2} \times \text{base} \times 4 , \text{ft} \]
To isolate the base, first multiply both sides of the equation by 2:
\[ 20 , \text{sq ft} = \text{base} \times 4 , \text{ft} \]
Next, divide both sides by 4 ft:
\[ \text{base} = \frac{20 , \text{sq ft}}{4 , \text{ft}} = 5 , \text{ft} \]
Thus, the base length of the yield sign triangle is 5 ft.