Note: I'm not sure what do you mean by "weight 0.05 kg/L". I assume it means the mass per unit of length, so it should be "0.05 kg/m".
Solution:
The fundamental frequency in a standing wave is given by

where L is the length of the string, T the tension and m its mass. If we plug the data of the problem into the equation, we find

The wavelength of the standing wave is instead twice the length of the string:

So the speed of the wave is

And the time the pulse takes to reach the shop is the distance covered divided by the speed:
On an approximate scale, A child breaths 20 times a minute as compared to only 12 to 16 in resting phase of an Adult.
So, In 60 minutes (1 hour), They breathe = 20 * 60 = 1200
In 24 hours (1 day), They breathe = 1200 * 24 = 28,800
In short, Your Answer would be: 28,800
Hope this helps!