18. If the average (arithmetic mean) of \[x\] and \[y\] is \[k\], which of the following is the average of \[x, y\] and \[z\]?
(A) \[\frac{2k + z}{3}\]
(B) \[\frac{2k + z}{2}\]
(C) \[\frac{k + z}{3}\]
(D) \[\frac{k + z}{2}\]
(E) \[\frac{2(k + z)}{3}\]
p3sec1.19
19. In the figure above, \[\Delta XYZ\] is equilateral, which of side of length 2. If \[WY\] is a diameter of the circle with center \[O\], then the area of the circle is
(A) \[\frac{\sqrt{3\pi }}{4}\]
(B) \[\frac{2\pi }{3}\]
(C) \[\frac{3\pi }{4}\]
(D) \[\pi \]
(E) \[\frac{3\pi }{2}\]

20. When 15 is divided by the positive integer \[k\], the remainder is 3. For how many different values of \[k\] is this true?
(A) One
(B) Two
(C) Three
(D) Four
(E) Five
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