I want to build a factory and I go to a bank for a loan, to finance part of the investment cost.  

Bankers tend to think in $a:b$: "How many euros we will lend for every euro the company will invest". And they tend to have rules of thumb on the matter, say a $3:1$ rule. From the point of view of the company, this could be written $1:3$ and here, confusion may arise more easily, because "$1/3$" should be interpreted as "the company will invest one third of what the bank will lend" and _not_ as "one third of the total cost of the factory". 

To arrive at this last magnitude the relation is always
 $a:b \rightarrow \frac {a}{a+b}$

More generally, I think a fraction $a/b$ (which then can be written also as a number, a decimal, etc), is meaningful only when $a$ and $b$ measure same entities in nature (in my example, money in the same currency).  But the concept represented usually by $a:b$ can bring together items that are not alike (say, "$a$ car-accident deaths per $b$ kilometers of highways), in which case there is no meaningful interpretation of $a+b$.