Approximation.
1 kg costs $12.5 and roughly equals 42 shrimps.
Each meal requires 5 shrimps and n meals are ordered daily, therefore 5n shrimps are required daily.
We need a dummy value for n in such a way that 5n is as close to 42 as much as possible.
Let n = 8
5n = 40
42 shrimps cost $12.5 so 40 must cost a little less, let's take it as $12.
n * p/q = 12 = 8 * p/q = 12
Now we just need to experiment with the values
If p = 2
Then 16/q = 12
and q is almost 1.33 but this isn't close to any whole number.
If p = 3
Then 12q = 24 and q = 2. Therefore, p = 3 and q = 2.
It also works if you take n = 9 but the calculation isn't that clean.
If 42 shrimps cost $12.5 then 45 must cost a little more, let's take it at $13.
n = 9
5n = 45
Daily cost = $13
n * p/q = 13
9 * p/q = 13
If p = 2
Then 18/q = 13 and q = 18/13 which approximates to 1.5 and not close to any whole number
If p = 3
9*3/q = 13
27/q = 13
q = 27/13 which approximates to 2.1 which is quite close to 2.
p = 3, q = 2
Yet another way is to increase the number of shrimps and cost to more round numbers
If 42 shrimps cost $12.5, how much would 50 cost? 50 is roughly 20% more than 42, so let's increase the cost per kg by 20% as well which equals $15.
Daily cost = 15
n = 10
10 *p/q = 15
p/q = 3/2
p = 3, q = 2