Lawrence Weinstein doesn’t know how many jelly beans are in this jar, but he has a very good guess. And it’s higher than you might expect. Weinstein, who teaches estimation at Virginia’s Old Dominion University, has a knack for solving problems with little data. His secret is more method than magic: Break questions into pieces, approximate, and use metric units for easier math.
Fermi estimation, as such a method is known, helps experts decide if problems— from jelly bean counts to carbon counts—warrant further calculation. Precision isn’t always necessary. Take sea level rise. By assuming the thickness of the Antarctic ice sheet (1,000 meters) and dividing that by how many Antarcticas he thought would cover the Earth (30), Weinstein surmised that melting ice caps could raise sea levels at least 30 meters. Though USGS reports suggest a 73-meter rise (80 meters if you include Greenland’s ice sheet), his rough guess still predicts catastrophe. “I don’t need to refine that number,” says Weinstein. “I’m in Virginia Beach. Either way, I’m underwater.” —Oliver Uberti
ANYONE’S GUESS You don’t have to split atoms to guess how many jelly beans are in this jar. Simply break the problem into steps.
1 Approximate the jar’s radius (r) in beans. (Hint: Count the jar's width, then divide by two.)
2 Estimate its height (h) in beans.
3 Use these numbers to figure the jar’s occupied volume: V = πr2 x h. Round π off to three.
4 Gloat (Put your mouse over the jar photo and wait for the answer.)
Photo: Mark Thiessen, NG Photographer