187. Julie went to the post office and bought both \( \$ 0.41 \) stamps and \( \$ 0.26 \) postcards. She spent \( \$ 51.40 \). The number of stamps was 20 more than twice the number of postcards. How many of each did she buy? 188. Jason went to the post office and bought both \( \$ 0.41 \) stamps and \( \$ 0.26 \) postcards and spent \( \$ 10.28 \). The number of stamps was four more than twice the number of postcards. How many of each did he buy? 189. Maria spent \( \$ 12.50 \) at the post office. She bought three times as many \( \$ 0.41 \) stamps as \( \$ 0.02 \) stamps. How many of each did she buy?

9. If \( x-y=4 \) and \( x y=21 \), then \( x^{3}-y^{3} \) is \( \begin{array}{ll}\text { (a) } 361 & \text { (b) } 316 \\ \text { (c) }-188 & \text { (d) } 360\end{array} \)

(a) 2023: If \( y \propto x \) and \( y=5 \) when \( x=15 \), Find the value of \( x \) when \( y=3 \) (b) If \( y \) varies inversely with \( x^{2} \) and \( x=3 \) when \( y=4 \), Find: [1] The relation between \( y \) and \( x \) [2] The value of \( x \) when \( y=9 \) (C) If \( a, b, c \) and \( d \) are proportion quantities, then prove that : \( \frac{a}{b+2 c}=\frac{c-a}{d-b} \)

Вычисли дискриминант квадратного уравнения: \( 5 \mathrm{x}-2+11 \mathrm{x}^{2}=0 \) Запиши в поле ответа верное число. \( \mathrm{D}= \)

\( 1 \frac{3}{2} \leqslant x<5 / 2 \) et \( \frac{2}{3} \leqslant y \leqslant \frac{4}{3} \) exarder \( x+y, x-y, x+y, \frac{x-3 y}{x} \)

b. \( \sqrt{1^{3}}+2^{1}+3^{3}+\cdots+n^{\overline{3}} \)