diff --git a/15.5_f.txt b/15.5_f.txt
new file mode 100644
index 0000000..1293911
--- /dev/null
+++ b/15.5_f.txt
@@ -0,0 +1 @@
+5 8 3 6 10 5 4 34 12 7 4 3 44 6 8 12 33 7 12 54 22 3 4
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diff --git a/15.5_g.txt b/15.5_g.txt
new file mode 100644
index 0000000..91e676c
--- /dev/null
+++ b/15.5_g.txt
@@ -0,0 +1 @@
+10 34 44 54 22 
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diff --git a/classwork9.py b/classwork9.py
new file mode 100644
index 0000000..d2cd527
--- /dev/null
+++ b/classwork9.py
@@ -0,0 +1,102 @@
+# Завдання 15.1
+def getValue(s, d):
+
+    assert 2 <= d <= 16, f'{d} is not in 2..16'
+
+    digits = ('0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f')
+    digit_val = [i for i in range(16)]
+
+    result = 0
+    for c in s:
+        if c.lower() not in digits:
+            print('stop!')
+            assert False, f'{c} is not correct digit for d = {d}'
+            return None    
+        index = digits.index(c)
+
+        if index >= d:
+            print('stop!')
+            assert False, f'{c} is not correct digit for d = {d}'
+            return None 
+
+        digit = digit_val[index]
+
+        result = result*d + digit
+        print('b = ', index,'n = ', result)
+
+    return result
+
+# Завдання 15.2
+def ln(x, eps):
+    assert abs(x) < 1, f'|x| < 1'
+    assert eps >= 0, 'epsilon should be positive'
+    y = 0
+    k = 0
+    while True:
+        y0 = y
+        y += (-1)**k * x**(k+1) / (k+1)
+        k += 1
+        if abs(y - y0) <= eps:
+            break
+    return y
+
+# Завдання 15.4
+def sum(*x, **y):
+    assert len(x) == len(y.keys()), f'size x not equal size y'
+    s = 0
+    for a,b in enumerate(y.keys()):
+        s += x[a]**2 + y[b]**2 + x[a]*y[b]
+    return s
+
+# Завдання 15.5
+def numbers_file(n, filename_1, filename_2):
+    try:
+        with open(filename_1) as f:
+            g = open(filename_2, 'w')
+            numbers = f.readline().split()
+            n_0, k = 0, n
+
+            while n < len(numbers):
+                g.write(f"{max(map(int, numbers[n_0:n]))} ")
+                n_0, n = n_0+k, n+k           
+            g.write(f"{max(map(int, numbers[n_0:]))} ")
+            g.close()
+            
+    except FileNotFoundError:
+        print(f'File {filename_1} not found')
+
+
+if __name__ == "__main__":
+    # 15.1
+    print(getValue('101', 2))
+    print(getValue('102', 3))
+
+    print(getValue('abc', 16))
+    print(getValue('819', 10))
+
+    s = input('Input number: ')
+
+    d = int(input('d = '))
+
+    n = getValue(s, d)
+
+    print('n = ', n)
+
+    # 15.2
+    y = ln(-0.5, 0.0001)
+    print(y)
+
+    y = ln(1, 0.0001)
+    print(y)
+
+    # 15.4
+    s = sum(2, 3, 4, y1 = 5, y2 = 6, y3 = 7)
+    print(s)
+    
+    s = sum(2, 3, y1 = 5, y2 = 6, y3 = 7)
+    print(s)
+
+    # 15.5
+    f = 'exercises/15.5_f.txt'
+    g = 'exercises/15.5_g.txt'
+    numbers_file(5, f, g)
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