Mathematics Practice Test

+9+-4 = 5

To solve this, we need to count the occurrences of 7 in ones place (10 times: 7, 17, 27, ..., 97) and in tens place (10 times: 70, 71, 72, ..., 79). In total, 7 appears 20 times. Therefore, the correct answer is 20.

The original solution has 4 kg of NaCl and 96 kg of water. Let \( x \) be the amount of water evaporated. The final amount of solution is \( 100 - x \), and the new concentration is \( \frac{4}{100 - x} = 0.05 \). Solving for \( x \) gives \( x = 20.83 \) kg, meaning 20.83% of water is evaporated.

Aristotle invented the syllogism as the keystone of his system of deductive logic. A syllogism is a form of reasoning in which a conclusion is drawn from two given or assumed propositions (premises). This system laid the foundation for formal logic and remained influential in Western philosophy and science for centuries. Aristotle's work on syllogisms was groundbreaking in its systematic approach to logical reasoning and inference.