Split an array in n elements and sum like below.

Question

This is different than previous uploaded question please see carefully.

I have one array called ans[i].

ans[i]={1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 } //n no.of inputed element.Not fix array

and i want to split in group of 3 and  stored it in array.Like this if x=2 the it should store the summation of 1+4+7+10+13..like below.

res[0]= 35 //1+4+7+10+13

res[1]=40 //2+5+8+11+14

res[2]=45 //3+6+9+12+15....like this.

 

 

 Please Help me