Hi saveyourlife,
As ex-l and others have said before the numbers are arbitrary values, so place no reliance on them whatsoever. However, to help you move forward with this so you can focus your energies elsewhere I'll provide a mathematical answer to your question.
The calculation is actually quite easy. All it is saying is that instead of 5,000 human years Destruction should occur after 5,000 Brahma years, where 1 Brahma Day has been defined by the young BK as 5,000 human years.
So 1 Brahma Year = 365 x 1 Brahma Day
So 1 Brahma Year = 365 x 5,000 human years = 1,825,000 human years = 1.825 million human years.
So 5,000 Brahma Years = 5,000 x 1 Brahma Year
So 5,000 Brahma Years = 5,000 x 1.825 million human years = 9,125,000,000 human years = 9.125 billion human years.
I deliberately excluded leap years in the calculation to make things easier.
The calculation by a young BK has two mathematical errors in it. I don't know if the mistakes are deliberate, designed to mislead and confuse, or are simply that of someone with limited mathematical ability. Also there is a mix up in terminology several times that helps in the confusion, but I'll try to be as clear as possible.
First error - each age is multiplied by 12 incorrectly (highlighted):
Satyug = 1,250 Brahma years = 1,250 x 12 x 365 = 5,475,000 Brahma Days
Tretayug = 1,250 Brahma years = 1,250 x 12 x 365 = 5,475,000 Brahma Days
Dwapar = 1,250 Brahma years = 1,250 x 12 x 365 = 5,475,000 Brahma nights
Kalyug = 1,250 Brahma years = 1,250 x 12 x 365 = 5,475,000 Brahma nights
I think this was supposed to represent the number of months in a year.
Number of days over Z years should be 365 days x Z years, not 365 days x 12 months x Z years.
So each age should be:
1250 years = 1250 x 365 days = 456,250 days.
So Satyug + Tretayug = 2 x 456,250 days = 912,500 days = Dwapar + Kalyug
Second error - this is more subtle, each part has been multiplied by 2,500 incorrectly (highlighted):
From Equation (1)
10,950,000 Brahma Days = 10,950,000 x 2,500 = 27,375,000,000 Human years
From Equation (2)
10,950,000 Brahma nights = 10,950,000 x 2,500 = 27,375,000,000 Human years
1 Brahma Day has been arbitrarily defined as 5,000 human years. So the calculation in each part should be Y Brahma days x 5,000 human years, not Y Brahma days x 2,500 human years.
The correct calculation, for each part would be, using the corrected value for the number of days:
Satyug + Tretayug = 912,500 days x 5,000 = 4,562,5000,000 human years = Dwapar + Kalyug
So Kalpa = 2 x 4,562,500,000 human years = 9.125 billion human years.
Again these numbers are arbitrary values and does not prove that Destruction will happen on any particular date. Hope this helps and clears things up so you can move on to other things outside of the BK.