A one page letter is folded into thirds. If you could repeat this tri-fold 20...

Let x=number of times the one page letter was folded.
Notice that thickness is defined, then, by t(x)=3x,measured in 1/150 in., since 150 pieces of stationery is 1 in.thick.
If this tri-fold was repeated 20 times, the nt(x)=320=3486784401 (1/150 inches)
=23245229.34 in=1937102.445ft=366.87546306818mi

Thickness $=3^x$
Thickness $=3^{20}=3486784401$
$\frac{3486784401}{150}=23245229$ inch
23245229 inch = 1937102 (1 inch = 0.083333 feet)
$\frac{1937102ft}{5280}=366.8$mi
Thickness = 366.8mi