First you need to try what are the requirements and how you can solve the problem? Try to figure out the steps you may use. then it will be easy to solve the problem.
This problem is also a vary simple one. Just need to think about how its working.
Run 2 for loops.say i and j.
First: Run loop i from 5 to 1. check i>=1
Second: Run loop j from 5 to 1. check j>=i
Third: print the value of i.
for(int i=5; i>=1; i--){
for(int j=5; j>=i; j--){
printf("%d",i);
}
printf("\n");
}