Finding n

[inky]

a1=5
d=10

The sum is 3000 (ie. How long will it take to fall 3000m?)

Formula I have is Sn=n/2 (2a + (n-1)d)

so,

3000=n/2 (10 + (n-1)10)

Trouble is, I can't work out what n is.

I know that, from putting the series into Excel - 5,15,25,... and using a simple formula to calculate the sums as the series progresses that when n=25 the sum is 3125. But I need to be able to do it 'properly'.

[Math Tutor]

[tex]3000=\frac{n}{2 }(10 + (n-1)10)[/tex]

[tex]3000=\frac{10n}{2 }(1 + (n-1))[/tex]

[tex]3000=5n*n[/tex]

[tex]600=n^2[/tex]

n=24.4948........