Firstly, let's find out the remaining number of mangoes after the rotten ones are removed. We start with a total of 620 mangoes and discard 45 rotten ones. This can be calculated with a simple subtraction equation:
620 (total mangoes) - 45 (rotten mangoes) = 575 (remaining good mangoes)
Next, we calculate the decrease in the number of mangoes:
620 (total mangoes) - 575 (remaining good mangoes) = 45 (the number of mangoes has decreased)
Lastly, we calculate the percentage decrease in the number of mangoes:
(Numerical decrease / Original number) * 100 = (45 / 620) * 100 = 7.26 (approximately)
So, the number of mangoes has decreased by 45, which represents a decrease of about 7.26%.