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curriculum/challenges/english/10-coding-interview-prep/project-euler/problem-430-range-flips.md

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+---
+id: 5900f51a1000cf542c51002d
+challengeType: 5
+title: 'Problem 430: Range flips'
+forumTopicId: 302101
+---
+
+## Description
+<section id='description'>
+N disks are placed in a row, indexed 1 to N from left to right.
+Each disk has a black side and white side. Initially all disks show their white side.
+
+At each turn, two, not necessarily distinct, integers A and B between 1 and N (inclusive) are chosen uniformly at random.
+All disks with an index from A to B (inclusive) are flipped.
+
+The following example shows the case N = 8. At the first turn A = 5 and B = 2, and at the second turn A = 4 and B = 6.
+
+
+
+Let E(N, M) be the expected number of disks that show their white side after M turns.
+We can verify that E(3, 1) = 10/9, E(3, 2) = 5/3, E(10, 4) ≈ 5.157 and E(100, 10) ≈ 51.893.
+
+Find E(1010, 4000).
+Give your answer rounded to 2 decimal places behind the decimal point.
+</section>
+
+## Instructions
+<section id='instructions'>
+
+</section>
+
+## Tests
+<section id='tests'>
+
+```yml
+tests:
+  - text: <code>euler430()</code> should return 5000624921.38.
+    testString: assert.strictEqual(euler430(), 5000624921.38);
+
+```
+
+</section>
+
+## Challenge Seed
+<section id='challengeSeed'>
+
+<div id='js-seed'>
+
+```js
+function euler430() {
+
+  return true;
+}
+
+euler430();
+```
+
+</div>
+
+
+
+</section>
+
+## Solution
+<section id='solution'>
+
+```js
+// solution required
+```
+
+</section>