A square with side $1$m can be divided into $25$ squares of side $20$ cm. Now Take number of points as pigeons, ie, 51, and number of small squares i.e., 25. as holes.
If you want to put $25 \times 2+1$ pigeons into $25$ holes then there will be at least one hole which contains $2+1=3$ pigeons.
Then there will be $1$ small square which can cover set of $3$ points.
.
please solve the problem using pegion hole principle
.