og cities

[pradeepkaushal9518]

plz explain i m confused

[kvcpk]

OG Explanation looks good for me.

30 cities will have 900 squares.

diadonal will have 30 squares without dots.
Hence 900-30 = 870

Now half will have dots and half do not.

870/2 = 435

[selango]

Note that 5 cities have 25 entries.

For 30 cities there ll be 900 entries.

30 entries along the diagonal are not needed?Why?

Because they are entries bw the same city like City A--City A,City B--City B

900-30=870

In 870 entries there ll be pairs of distance bw same cities[for ex city A--City B and City B]

there should be only one entry bw 2 city.

-->870/2=435 entries.

You can easily understand if u apply this concept in the diagram shown.



Pick B

[Vipulvp]

plz explain i m confused

You can probably start small. In case of three cities, we have three entries. In case of four cities, we need six entries. In case of five cities, we need ten entries. If we are able to see the pattern, we can easily apply it to thirty cities.
3 --> 3C2 (=3)
4 --> 4C2 (=6)
5 --> 5C2 (=10)
Hence 30 --> 30C2 = 435