An opinion is reasonable when it is both facts-based and a rational conclusion from the facts. A lack of facts, or an invalid line of reasoning from the facts, reduces the opinion to "empty opinion" or "unreasonable opinion" respectively.
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That is the correct spelling of the word "reasonable."
Estimate the answer. If the calculated answer is close to the estimate then it is reasonable.
12 centimetres is a reasonable estimate.
One adjective from the word "reason" is "reasonable."
Whether or not it is reasonable depends on the context - which has not been provide.