no of vertices =n
min degree for each vertex =3
now total degree =3*n
no of edge =25
now we know for undirected graph ,
2*edge = sum of degree
sum of degree =2*25 =50
now 3*n = 50
n= 16.67
now if someone confused which one to took ceil of floor
now just think if we put n= 17 then 3*17 =51 which is not the sum of degree as it is 50 and we can’t say 16 vertex of degree 3 and one vertex of degree 2 as min degree should be 3.
now if you choose 16 then 16*3 =48 which is also not equal to 50 also but here is the possibility that we have 15 vertex of degree 3 and one vertex of degree 5 as 5 is greater than min degree 3 .so ,3*15 +5 =50 .
thats why we took 16 as answer.