This is just a trial and error problem:

 <-- too small
 <-- too small
 <-- too small, but fairly close to 68
 <-- way too big.

So the closest is t=3.

If you were studying logarithms, you'd be able
to approximate the answer very closely as 3.043731421.

But that's not much bigger than 3, so 3 is an approximate 
answer.

Edwin