Let's see, no dollars here but the four £20 notes in my wallet, folded three times, are just 3.9mm thick. That's 4x8=32 notes, so one note is 0.122mm thick.

The exchange rate has gone wild lately, but is currently about $1.57 to the pound, so $40b is about £25.5b. Divide that by 20, multiply by 1.22 and you get 1554140 metres, or 1554 kilometres. That's 970 miles, from my house to Warsaw, or from Washington to Miami.