Eve Does Some Magic with Nothing Up Her Sleeve

Right. Eve The Magician asks the audience for a value of G, and Carol shouts out “5”. Eve then asks Alice to think of a number (a) and multiply the two values together to give y:

y = aG

“Don’t tell me the result”, says Eve. She now asks for another number from the audience, and they shout out “7”. Eve writes something on a piece of paper, and shouts out that…