POH5で書いたC#のコードを公開。
所要時間は、1問目と2問目は数分。レナが40分ほど、ミナミは25分ぐらい。

###1問目
修正したけど変数名のoddとevenを間違えていたのはここだけの秘密。てへ

public class Hello{
    public static void Main(){
        var line = System.Console.ReadLine();

        var even = true;
        foreach(var c in line) {
        	if(even) System.Console.Write(c);
        	even = !even;
        }
        System.Console.Write(System.Environment.NewLine);
    }
}

##2問目

public class Hello{
    public static void Main(){
        var n = System.Convert.ToInt32(System.Console.ReadLine());
        int[] totals = {0,0,0,0,0,0,0};
        for(var i=0; i<n; i++) {        	
        	totals[i%7] += System.Convert.ToInt32(System.Console.ReadLine()); 
        }
        foreach(var total  in totals) {
	        System.Console.WriteLine(total);
        }
    }
}

##レナ

using System;
public class Hello{
    public static void Main(){
        var line = Console.ReadLine();
        var lineFields = line.Split(' ');
        var x = Convert.ToInt32(lineFields[0]); 
        var y = Convert.ToInt32(lineFields[1]); 
        var N = Convert.ToInt32(lineFields[2]); 

        int[,] array = new int[x, y];

        for(var yy=0; yy < y; yy++) {
            var data = Console.ReadLine();
            var dataFields = data.Split(' ');
            for(var xx=0; xx < x; xx++) {
                array[xx, yy] = Convert.ToInt32(dataFields[xx]);
            }
        }

        bool[,] table = new bool[x, y];
        for(var i=0; i < N; i++) {
            var select = Console.ReadLine();
            var selectFields = select.Split(' ');
            var xs = Convert.ToInt32(selectFields[0]); 
            var ys = Convert.ToInt32(selectFields[1]); 
            var xe = Convert.ToInt32(selectFields[2]); 
            var ye = Convert.ToInt32(selectFields[3]); 
            
            for(var tx=xs; tx<=xe; tx++) {
                for(var ty=ys; ty<=ye; ty++) {
                    table[tx-1, ty-1] = true;
                }
            } 
        }            

        var sum = 0; 
        for(var yy=0; yy < y; yy++) {        	
            for(var xx =0; xx < x; xx++) {
                sum += table[xx, yy] ? array[xx,yy] : 0;
            }
        }
	    
        System.Console.WriteLine(sum);
    }
}

##ミナミ

using System;
public class Hello{
    public static void Main(){
        var line = Console.ReadLine();
        var lineFields = line.Split(' ');
        var x = Convert.ToInt32(lineFields[0]); 
        var y = Convert.ToInt32(lineFields[1]); 

        int[] array = new int[x];

        for(var i=0; i < y; i++) {
            var data = Console.ReadLine();
            var fields = data.Split(' ');
            
            for(var j=0; j < x; j++) {
               if(fields[j] == "1") array[j]++; 
            }
        }

        for(var i=0; i < y; i++) {
            var outLine = String.Empty;  
            for(var j=0; j < x; j++) {
                if(y-i <= array[j]) {
                    outLine += "1 ";
                } else {
                    outLine += "0 ";
                }
            }
            Console.WriteLine(outLine.TrimEnd());
        }

    }
}