The Tcl interpreter itself does not evaluate math expressions. Tcl just does grouping, substitutions and command invocations. The expr command is used to parse and evaluate math expressions.
expr 7.2 / 4 => 1.8
The math syntax supported by expr is the same as the C expression syntax. The expr command deals with integer, floating point, and boolean values. Logical operations return either 0 (false) or 1 (true). Integer values are promoted to floating point values as needed. Octal values are indicated by a leading zero (e.g., 033 is 27 decimal). Hexadecimal values are indicated by a leading 0x. Scientific notation for floating point numbers is supported. A summary of the operator precedence is ...