what is the default order and direction of operation? I have a division like this 

16/8/4/2 

what is the default way to do calculations when the bracket is not specified .
Method 1 :
Is it correct to go from right to left like

[16/ (8 / { 4 / 2 } )]

I tried it in calculator as 16/8/4/2 and it gave 0.25 
Method 2 :
May be it went from left to right

[({16 / 8} / 4 ) / 2]

Why I am asking this is because I saw a problem in GRE book
like

ab/c/cd/a 

without any brackets .
their solutions reads
Method 3 :
$$ (\frac{ab}{c} \times \frac{a}{cd} ) $$
$$ = \frac{a^2 \times b}{c^2 \times d} $$
I am puzzled how they assumed brackets like that without any indication in question .
what is the right way to go as per maths .
Attaching image to show the exact problem . (to show the bars even look of the same size)


From the image I would say they mean
$$
ab/c\big/cd/a=\frac{\frac{ab}{c}}{\frac{cd}{a}}=\frac{ab}{c}\frac{a}{cd}=\frac{a^2b}{c^2d}.
$$
But you are right, without brackets we have several possibilities, e.g., $16/8$ divided by $4/2$ is $1$.
