Help me understand recursion please

Question

Example 1:

def fun(a):
if a > 30:
return 3
else:
return a + fun(a + 3)

print(fun(25))

Example 2:

def fun(a):
if a > 30:
return 3
else:
return a + fun(a + 3)

print(fun(20))

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Why is example 1 as easy as 3 + 25 + 3 + 25 to get the answer 56 but the same method doesn’t apply for example 2. I get the output 101 instead of what I thought would be 46.

Answer 1

Follow through the if statement in stacks and you get;
20 + (20 + 3) + (23 + 3) + (26 + 3) + (29 + 3) + 3

Answer 2

The 2nd program works as follows:

It first calculates fun(20):
return 20 + fun(23)
| Now the program calculates fun(23):
| return 23 + fun(26)
| | Now it calculates fun(26):
| | return 26 + fun(29)
| | | Now it is fun(29):
| | | return 29 + fun(32)
| | | | fun(32) returns 3 directly
| | | return 29 + 3
| | return 26 + 32 (because fun(29) = 32)
| return 23 + 58 (because fun(26) = 58)
return 20 + 81 (because fun(23) = 81)