Вот решение на C#
using System;
using System.Collections.Generic;
using System.Linq;
namespace Help
{
class MainClass
{
public static void Main(string[] args)
{
int day = 1;
double mass = 50;
double changing = 1.2;
while(mass < 120)<br>
{
mass *= changing;
day++;
}
Console.WriteLine($"В {day} день");
}
}
}