If the natural numbers are divided into groups {1}, {2,3}, {4,5,6}, {7,8,9,10}…… then the sum of 50^th group is ……

1^st term = 1 element 
2^nd term = 3 elements
3^rd term = 6 elements
i.e  $\sum 1,\sum 2,\sum 3,\mathrm{.....}\sum 50$
The last term of 50^th group is 1275
First term : 1275-50+1=1226=1226+1227+1228+….+1275 $=\frac{n}{2}(a+l)=62525$