If you spread a billion dollars worth of 1-dollar bills over the ground, how many square miles would they cover? (The length and the width of a 1-dollar bill are 6.14 and 2.61 inches, respectively.)

We have length and width of 1-dollar bill

length : $l=$$6.14$ inch

width : $w=2.61$ inch

Area of 1-dollar bill is given as 

$A=l\times w$

$A=$$6.14\times 2.61$ $i{n}^2$

$A=$$16.0254$  $i{n}^2$

Number of 1-dollar bills =1 billion 

1 billion = ${10}^9$

Area of 1billion dollar bills will be 

$A_1=A\times {10}^9$

$A_1=$$16.0254\times {10}^9$ $i{n}^2$

Using conversion unit

1 mile = 63360 inches

1 inch = $\frac{1}{63360}$ mile

$1$ $i{n}^2=$${\left(\frac{1}{63360}\right)}^2$ $m{i}^2$ 

1 $i{n}^2=$$\frac{1}{4014489600}$ $m{i}^2$

So, 

$A_1=$$\frac{16.0254\times {10}^9}{4014489600}$  $m{i}^2$

$A_1=$$\frac{160254000}{4014489}$ $m{i}^2$

$A_1=3.99188$$97$ $m{i}^2$

Rounding off two decimal places 

$A_1=3.99$ $m{i}^2$

Area of 1 billion dollar bills = 3.99 square miles