A popular economics joke has a physicist, a chemist, and an economist all stranded on a desert island. There is one can of food, but nothing to open it with. The physicist presents his proposed solution (usually hitting the can with a rock). The chemist presents his proposed solution (such as heating a seawater solution with the sun’s rays). The economist comes up with the simple, unrealistic solution: “Assume we have a can opener…” One variant is: “Assume the can is open…”
 
The desert island “assume we have a can opener” joke dates to at least 1970 and possibly originated with British economists.
 
   
Economcis Jokes
A physicist, a chemist and an economist are stranded on an island, with nothing to eat. A can of soup washes ashore. The physicist says, “Lets smash the can open with a rock.” The chemist says, “Let’s build a fire and heat the can first.” The economist says, “Lets assume that we have a can-opener…”
   
Google Books
Economics as a Science
By Kenneth E. Boulding
Bombay : Tata McGraw-Hill
1970
Pg. 101:
The physicist and the chemist each devised an ingenious mechanism for getting the can open; the economist merely said, “Assume we have a can opener”!
       
Google Books
Punch
Volume 259
1970 (Google Books date might not be accurate—ed.)
Pg. 457:
Two desks away, a leading British economist was telling a joke. “A chemist, a physicist and an economist were on a desert island,” he began.
(...)
“The economist said he couldn’t understand all the fuss. The answer was perfectly simple. Let us assume we have a tin opener.”
 
13 February 1973, Wall Street Journal, “Speaking of Business” by Lindley H. Clark Jr.:
“Let’s assume we have a can opener.”
 
Google Books
19 September 1974, New Scientist, pg. 776, col. 2:
(Incidentally last week’s joke of Denis Healey;s deserves repeating: the chemist, the engineer, and the economist cast up on a desert island with only a can of corned beef. The chemist thought of dissolving the can but could find no reactive substances. The engineer tried bashing the can with stones. The economist, when it came to his turn, said, “Let us assume we have a tin-opener.”)
 
Google News Archive
21 May 1977, Financial Post (Toronto), “Stop Me If…,” pg. 6, col. 1:
Three men found themselves stranded together on a desert island, with their only food being a quite firmly closed can of beans. The three—engineer, chemist, and aconomist—repaired to separate beaches to deliberate how to get the can open. A short while later, they rejoined.
 
The engineer, having estimated the tensile strength of the steel can and having calculated inside and outside pressure points, proposed they throw the can against the cliff until the can opened.
 
The chemist suggested scratching the can lid with a rock, putting seawater on the scratches, then leaving the can in the sunlight until the lid corroded open.
 
They turned to the economist, who, squinting at the sky, commenced, “Assume we have a can-opener…”
 
22-29 December 1977, New Scientist, pg. 775, col. 1:
A chemist, a physicist and an economist are marooned on a desert island without food. Suddenly they discover a cache of canned goods but there is no opener. The chemist begins looking about for chemicals in their natural state so he can make up a solution which will dissolve the tops of the cans. The physicist picks up a rock and begins calculating what angle, what force, what velocity he will need to strike the can with the rock in order to force it open. The economist merely picks up a can and says,“Let us assume this can is open.” (In a variant, “Let us assume we have a can opener.”)