Can anyone explain the logic for the following code in python

Question

def sub(x,y):
return x-y
def mult(x,y):
return x*y
def pr(f1, f2, d):
print(f1(f2(d+1, d), d))
pr(mult, sub, 4)

answer is 4

Answer 1

In this code

function pr(f1, f2, d) = pr(mult, sub, d)

f1 refer to mult function
f2 refer to sub function
d refer to 4

So for f2 : sub will be (x - y) = (d+1-d) = 1

So f2(d+1, d) = 1

Now print(f1(1, d))

So for f1 : mult will be (x*y) = 1*d = 1*4 = 4

so now f1(1, d) = f1(1, 4) = 4

now print(4) = 4

Answer 2

look at line 6
first. subtraction 5 - 4 = 1.
then multiplication 1*4 = 4
then you print.