The probability that two events will both happen is 
, where 
is the probability that the first event will happen, and 
is the probability that the second event will happen when the first event is known
to have happened.
Fermat's Principle of Conjunctive Probability
See also
Conditional ProbabilityExplore with Wolfram|Alpha
References
Whittaker, E. T. and Robinson, G. The Calculus of Observations: A Treatise on Numerical Mathematics, 4th ed. New York: Dover, p. 317, 1967.Referenced on Wolfram|Alpha
Fermat's Principle of Conjunctive ProbabilityCite this as:
Weisstein, Eric W. "Fermat's Principle of Conjunctive Probability." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/FermatsPrincipleofConjunctiveProbability.html