def grades(*args):
 
	# this checks for invalid grades
	invalid = [str(y) for y in args if not (0 <= y <= 100)]
	if invalid:
		raise ValueError('invalid grades entered: ' + ', '.join(invalid))
 
	# puts passing grades in a list
	passing = [x for x in args if x >= 50]
 
	# return 0.0 if no passing grades are found; else average out the rest
	return 0.0 if not passing else sum(passing)/len(passing)
 
print(grades(10, 30)) # 0.0
print(grades(60, 30)) # 60.0
print(grades(60, 80)) # 70.0
print(grades(70, 80, 90)) # 80.0
 
print(grades(100, 120)) # raises error

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