To determine which of the sums results in a rational number, let's analyze each option:
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\( \sqrt{36} + \sqrt{221} \)
- \( \sqrt{36} = 6 \) (a rational number)
- \( \sqrt{221} \) is not a perfect square, so it is an irrational number.
- The sum \( 6 + \sqrt{221} \) is irrational.
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\( 0.643892553... + \frac{3}{4} \)
- \( \frac{3}{4} \) is a rational number.
- \( 0.643892553... \) is a decimal which appears to be non-terminating and non-repeating, indicating it is irrational.
- The sum of a rational and an irrational number is irrational.
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\( \pi + \sqrt{4} \)
- \( \sqrt{4} = 2 \) (a rational number).
- \( \pi \) is an irrational number.
- The sum \( \pi + 2 \) is irrational.
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\( 0.6\overline{6} + \frac{9}{10} \)
- \( 0.6\overline{6} \) is equal to \( \frac{2}{3} \) (a rational number).
- \( \frac{9}{10} \) is also a rational number.
- The sum \( \frac{2}{3} + \frac{9}{10} \) is rational.
Now, between the two options that result in rational numbers, the ones we found were \( 0.6\overline{6} + \frac{9}{10} \) and \( \frac{3}{4} + 0.643892553...\) (which we concluded is not a rational result).
Thus, the sum that results in a rational number is:
\( 0.6\overline{6} + \frac{9}{10} \)