How answer is 49 ?can anyone explain it to me

This chart may seem confusing, but it's not too bad once you get the hang of it. It is known as a mileage chart and it shows the distances between any two cities of this current list.

For example, the distance between Bedford and Cambridge is 29 miles because of the "29" in the very top block. This is the intersection between the "bedford" column and the "cambridge" row. Another example: The distance from Cambridge to Haverhill is 48 miles. Look in the "cambridge" column and the "haverhill" row.

With this in mind, we just need to look at the right rows and columns to find the distances to add up. See the set of attached images below. Figure 1 shows me marking the "cambridge" column and the "royston" row. In this row and column combo is the number 13. So the distance between these two cities is 13 miles.

Then onto figure 2 which shows the distance between Royston and Huntingdon is 21 miles. Figure 3 shows the distance from Huntingdon to Cambridge is 15 miles.

Adding the distances gives: 13+21+15 = 49

So that is why the total round trip distance is 49 miles.