C :

#include<stdio.h>
void main()
{
	int n;
	double p;
	scanf("%d",&n);
	if(n>=30){ 
		p=n;
 	}else if(n>=20&&n<=29){
 		p=1.2*n;
	}else if(n>=10&&n<=19){
		p=1.5*n;
	}else{
		p=1.8*n;
	}
	printf("%.1f",p);
} 

C++ :

#include <bits/stdc++.h>
using namespace std;
int main(){
   int a;
   double b;
   cin>>a;
   if(a >= 30){
   	  b = a * 1;
   }else if(a >= 20){
   	  b = a *1.2;
   }else if(a >= 10){
   	  b = a * 1.5;
   }else{
   	  b = a * 1.8;
   } 
   cout<<fixed<<setprecision(1)<<b<<endl;
   return 0;
}

Python :

n=int(input())
if n>=30:
    s=n
elif n>=20:
    s=n*1.2
elif n>=10:
    s=n*1.5
else:
    s=n*1.8
print('%.1f'%s)