Final answer:
The best replacement for the underlined word is 'methods' as it accurately conveys the idea of a systematic approach to solving problems.
Step-by-step explanation:
The best replacement for the underlined word in the sentence would be methods. In this context, 'formulas' are specific methods or procedures that scientists use to arrive at conclusions. Other options like 'tools', 'books', and 'calculations' do not accurately convey the idea of a systematic approach to solving problems. By using 'methods', the sentence maintains its meaning and coherence.
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