Estimate the number of dollar bills (15.5 cm wide), placed end to end, that it would take to circle the Earth (radius = 6.40 × 103 km).

 #_bills = 2.59 10⁸  Bills

Explanation:

For this exercise we must calculate the length of the Earth's circle

        L = 2π r

Let's reduce to the SI system

        r = 6.40 10³ km (1000m / 1km) = 6.40 10⁶ m

        d = 15.5 cm (1m / 100cm) = 0.155 m

        L = 2π 6.40 10⁶

        L = 40.21 10⁶ m

now we can use a direct rule of proportions (rule of three). If 1 bill has d = 0.155, how much bill will have L

       #_bills = 1 L / d

       #_bills = 1 40.21 10⁶6 / 0.155

       #_bills = 2.59 10⁸  Bills

the total number of bills is  2.6 10⁸ bills

need to see circuit to get time constant