Is 3.5 the same as 3 1/2? i need to give an example

Yes this is the same because 1/2 is equivalent to 0.5 an 2/2 is 1 so 1 divided by 2 is 0.5 the same as 1/2 so 3 1/2 does equal 3.5

Yes.
The decimal 0.5 can be represented as a fraction of 5/10, which simplifies to 1/2.
0.5 is half of 1, as is 1/2.
When you multiply either 0.5 or 1/2 by 2, you get 1.
So when you add 0.5 or 1/2 to the same base number, they both equal the same thing.

3.5 does equal 3 ½

Hope this helps!

When i did the math i got 11 cm