OG writing question

Test Preparation
1

<p>The young fish were very tiny, yet each of them ate many times its own weight in solid food every day. </p>

<p>Answer is "No Error". But how is "in" explained grammatically? Is there anything like "eat in solid food" exiting??? I don't think so...</p>

<p>btw, i didn't have the prob with "its", i know it refers to "each". what i couldn't understand here is the usage of "in", so could anyone explain that? thanks!</p>

2

<p>“in” is used to denote units or types in phrases involving measurements.</p>

<p>Examples — (1) He paid his bill in gold. (2) His paid his bill in kind. (3) He paid his bill in goods. (4) He painted in ink. (5) He measured her height in inches.</p>