7.2 Write a program that prompts for a file name, then opens that file and reads through the file, looking for lines of the form: X-DSPAM-Confidence: 0.8475 Count these lines and extract the floating point values from each of the lines and compute the average of those values and produce an output as shown below. Do not use the sum() function or …

py4e_ex_07_02

# Use the file name mbox-short.txt as the file name
fname = input("Enter file name: ")
fhand = open(fname)

count = 0
for line in fhand:
    if line.startswith("X-DSPAM-Confidence:") : 
        count = count + 1

total = 0
for line in fhand:
    if line.startswith("X-DSPAM-Confidence:"):
       line = float(line[21:])
       total = line + total
       
average = total/ count

print("Average spam confidence:",average)

Use the file name mbox-short.txt as the file name

fname = input("Enter file name: ")
fhand = open(fname)

count = 0
for line in fhand:
if line.startswith("X-DSPAM-Confidence:") :
count = count + 1

total = 0
for line in fhand:
if line.startswith("X-DSPAM-Confidence:"):
line = float(line[21:])
total = line + total

average = total/ count

print("Average spam confidence:",average)

In the above code the logic in the line i couldn't got can anyone explain me please?
line = float(line[21:])
total = line + total
how 21 ?