Which is the best definition of mathematical proof?

Which expression calculates the speed in meters per second of an object that travels a distance of 100 m every 20 seconds

Successful implementation of a new system is based on three independent modules. module 1 works properly with probability 0.96. for modules 2 and 3, these probabilities equal 0.95 and 0.90. compute the probability that at least one of these three modules fails to work properly.

Divide the following fractions 3/4 ÷ 2/3 1/2 8/9 9/8 2