A basic rule of statistics:

The probability for both A and B to happen at the same time is:

P(A and B) = P(A) * P(B)

Now imagine a project having 5 different moving parts, and we are able to get each part working 90% of the times. The probability of getting all parts to work together at the same time is:

P(working project) = 0.9 * 0.9 * 0.9 * 0.9 * 0.9 = 0.59049

That’s a mere 59% chance.