You can use the IF statement.

IF In_Bool 1 THEN
Out := 1.0;
ELSIF In_BOOL 2 THEN
Out := 2.0;
// and so on…

ELSE // no input active
Out := 0.0;
END_IF;

I guess it may be an exercise and then task will be changed to “any bit can be 1”
in such case you can use 
Out:=0.0;
IF In_Bool_1 THEN
Out:=Out+1.0;
END_IF;

IF In_Bool_2 THEN
Out:=Out+2.0;
END_IF;

IF In_Bool_3 THEN
Out:=Out+4.0;
END_IF;
 

and so on