Subtract 5 from both sides
Case 1: Probabilities cannot add up to a number greater (or less) than 1. This would mean there is greater than a 100% chance of something happening which just doesn't make sense. 0.4 + 0.4 + 0.3 = 1,1
Case 2: You cannot have a negative probability. That is claiming that there is a -10% chance of an event happening, there is at the very least a 0%. Despite them "adding" up to 1, the negative probability makes no sense.
Hope I helped!
It would take 70,000 seeds if five out of seven are good. So over a quantity of 50,000 plants desired, you would sow 70,000 seeds. As 20,000 would statistically fail.