3The Rule of Three

the value for "x" is 125

History facts
a/b = c/d
The Rule of Three was a shorthand version for a particular form of cross multiplication, often taught to students by rote. This rule was known to Indian (Vedic) mathematicians in the 6th century BCE and Chinese mathematicians prior to the 7th century CE, though it was not used in Europe until much later.

by João Santa Cruz, Lisbon 2013