12+((1.0*5)/7) is the correct version, notice the parentheses around 1.0*5.
PS: 12+(5/(7 * 1.0)) is valid too. If you want the result of a division to be a floating point value, you just have to make sure one of the values is a floating point value.
Not sure why there is no IntToFloat function btw, this would be probably faster than "* 1.0". I'll add it to the wish list.