Does anybody know how to write 45/70 in simplest

To reduce a fraction to a simpler form, find a common factor of the numerator and denominator, and divide them both by it.  If they have no common factor, then the fraction is in its simplest form.

Just looking at 45 and 70, we know immediately that they're both multiples of 5 .
So let's divide top and bottom by 5 :

45/70 = 9/14 .

That's simpler. 

Now, do 9 and 14 have any common factors ? No ?  Good.
That means that  9/14 is the simplest form of 45/70.

You didn't have to know what it is. You just had to know how to find it.