RMarkdown collapsible panel

Another simple solution that would work (but without buttons and styling).

```{r, eval=FALSE}
hist(1:10)
```

<details>
  <summary>Click for Answer</summary>
    ```{r, echo=FALSE, eval=TRUE}
    hist(1:10)
    ```
</details> 

And here are the two states:

Collapsed

Expanded

You can use multiple tabs (add {.tabset} after the header). It's very simple to generate them using r-markdown and they look almost the same as collapsible panel (of course you need to have more than one option).
Not to paste same code multiple times specify code argument in chunk options (code = readLines("code.R")). Or you can have only one panel for code and answer so you wouldn't need external document.

---
title: Collapsible Panel
output:
  html_document:
    theme: flatly
    highlight: tango
---

# Question 1 {.tabset .tabset-fade .tabset-pills}

## Question

How does uniform distribution look like?

## Code 

```{r, echo = TRUE, eval = FALSE, code = readLines("Q1.R")}
```

## Answer

```{r, echo = FALSE, eval = TRUE, code = readLines("Q1.R")}
```

Code file (Q1.R):

hist(1:10)

---

To not have any content and then show answer you can make first tab completely empty with:

# Question 1 {.tabset}

##  <span>&#8203;</span>

## Answer

```{r, echo = FALSE, eval = TRUE, code = readLines("Q1.R")}
```