different between n+1 and n in range python code example

Example 1: range parameters python

arr_data=[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20] 
user = int(input("Enter the product of numbers: ")) 
for i in range(0,20,1): 
    a = arr_data[i] 
    for t in range(0,20,1): 
        b = arr_data[t] 
        if (a*b) == user: 
            print(a,"x",b,"=",user) 
        else: 
            pass

Example 2: class range python code

class Range:
   def __init__(self, start, end):
      self.setStart(start)
      self.setEnd(end)
   def getStart(self):
      return self.start
   def setStart(self, s):
      self.start = s
   def getEnd(self):
      return self.end
   def setEnd(self, e):
      self.end = e
   def getLength(self):
      return len(range(self.start, self.end))
   def overlaps(self, r):
      if (r.getStart() < self.getEnd() and r.getEnd() >= self.getEnd()) or \
         (self.getStart() < r.getEnd() and self.getEnd() >= r.getEnd()) or \
         (self.getStart() >= r.getStart() and self.getEnd() <= r.getEnd()) or \
         (r.getStart() >= self.getStart() and r.getEnd() <= self.getEnd()):
          return True
      else:
          return False