Thank you all for your valuable reply,

One solution what I applied already was
###
def add()
 a=5
 b=6
 return a+b
end

def sub()
 a=5
 b=6
 return a-b
end

def pro()
 a=5
 b=6
 return a*b
end

loop do

  printf "Enter \n
          1-Addition\n
          2-Subtraction\n
          3-Multiplication\n
          4-Exit\n : "

    a=STDIN.gets
    b=a.to_i

   case b

    when 1
      add()

    when 2
      sub()

    when 3
      pro()

    when 4
      exit

    else
      puts "Wrong Entry"
  end
end

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