Dimension values

To be honest, I don’t believe that Creo accepted 

3 1/2 * 2 , try (3+1/2)*2

There is a setting for the display of inch values in drawings, and how exact the display for 1/2 or eg. 5/8 should be displayed as far I know. 

When your dimensions are fractional, when you go to edit them, you will notice it puts a plus sign (+) between the number and the fraction. This is how you have to enter your simple equations also. I typically just use decimals when doing simple equations just to avoid the extra plus sign.

And creo doesn't follow math rules for order of operations, so you need to use parenthesis if you need to control order of operations. For me, if it's more than simple, I'll do the math somewhere else.