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3 problems match “rational numbers”. Clear

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1. Sum of four rational numbers ●●○○○ 20 ptsAdd $2\frac{3}{5}$, $\frac{-15}{13}$, $\frac{-13}{-15}$ and $-4\frac{3}{5}$. Round the sum to two decimal places.
2. Position of a fraction in a sequence ●●●○○ 50 ptsThe positive rational numbers are arranged in the order $\frac{1}{1}, \frac{2}{1}, \frac{1}{2}, \frac{3}{1}, \frac{2}{2}, \frac{1}{3}, \frac{4}{1}, \frac{3}{2}, \frac{2}{3}, \frac{1}{4}, \cdots$. Find the position number of the number $\frac{1989}{1949}$ in this sequence.
3. Minimal period of a sum of decimals ●●●●○ 100 ptsPositive rational numbers $a$ and $b$ are written as decimal fractions, each of which has a minimal period of 30 digits. The decimal representation of $a-b$ has a minimal period of length 15. For what smallest natural number $k$ can the decimal representation of $a+kb$ also have a minimal period of length 15?