Why does the solution for t in the equation Vf=Vi+at result in a negative sign?

[vysero]

Ok so here is my problem:

Vf=Vi+at

I want to solve for t so:

(Vf-Vi)/a = t

right so now my problem starts how can this happen:

t = -Vf/a ?

The solution above comes from this site http://www.monmsci.net/~fasano/phys1/Chapter_2_10.pdf

scroll down to problem 2.31 and you will find what I am looking at, in this situation Vi = 0 so they disregarded it and somehow during that process (at least i believe it was during that process) they came up with:

t = -Vf/a

Please explain to me how that works, where the heck did the - sign come from??

 

[jedishrfu]

looks like a typo to me. sometimes in gravity problems +v means its going up vs -v means its falling but that's not the case. notice in the actual answer the - is gone.

 

[vysero]

Oh well I didn't even see that lol well thank you it prob. was just a typo