The distance, s = 40t₁ where t₁ is the time to complete the outward journey.
The same distance = vt₂ where t₂ is the time to complete the return journey at speed v.
For the whole journey, 2s=60(t₁+t₂). Therefore s=30(t₁+t₂)=40t₁=vt₂.
30t₁+30t₂=40t₁, 10t₁=30t₂, t₁=3t₂, so s=120t₂=vt₂ and v=120kph.
Given the dataset

We start by computing the average:

We compute the difference bewteen each element and the average:

We square those differences:

And take the average of those squared differences: we sum them

And we divide by the number of elements:

Finally, we take the square root of this quantity and we have the standard deviation:

Socratic is a really good app for these type of questions! I definitely recommend. For science too!
35, 42, and 49 jagskahskahslahslabajak
Your answer would be 124.559