Brain Out Level 115

Overview

The prompt asks "What's the answer?" and lists a number pattern: 1 equals 5, 2 equals 15, 3 equals 215, 4 equals 3215, and 5 equals question marks. An Enter field and an OK button sit below the sequence. The pattern builds each answer by placing the previous input digit in front of the previous result, so the fifth entry adds a 4 at the front of 3215. The resulting value is 43215, typed into the field and confirmed with OK.

• Difficulty: Regular
• Estimated moves: 1

Step-by-Step Solution

1. Read the listed equations in order: 1 = 5, 2 = 15, 3 = 215, 4 = 3215, 5 = ??.
2. Compare the second result to the first: the result for 2, 15, puts the digit 1 in front of the result for 1, which was 5. The trailing 5 from the first line stays at the end of the second result.
3. Compare the third result to the second: 215 puts the digit 2 in front of the previous result 15. Again the older 15 remains intact behind the newly added leading digit.
4. Compare the fourth result to the third: 3215 puts the digit 3 in front of the previous result 215. This same prefixing pattern repeats across all three later lines rather than changing direction.
5. State the rule clearly: for input n, the result starts with n minus one, followed by the entire previous result. The number of digits grows by one with each line, and the final 5 never moves from the end.
6. Apply it to input 5: place the digit 4 in front of the previous result 3215, giving 43215. Read the built string back as four, three, two, one, five to confirm the order.
7. Tap the Enter field, type 43215, and press OK. The level accepts 43215 as the next value in the sequence. If the entry is rejected, recheck that the leading digit is 4 and that the older 3215 was kept intact behind it.

Strategy & Reasoning

The pattern is built by prefixing rather than by arithmetic between the numbers. Each line adds one new leading digit to the string from the line above, while the trailing 5 stays fixed at the end. Reading the results as digit strings makes the construction visible: 5, then 15, 215, 3215, and finally 43215. Trying to multiply, subtract, or divide the displayed numbers leads to values that do not match the given examples.

A related riddle treats 1 equals 5 as a reversible statement and answers 5 equals 1, but this version of the level follows the visible prefix sequence and expects the five-digit continuation. The four listed examples already establish the prefix rule clearly, so the safest check is to confirm the leading digit and the preserved tail before typing. Once 43215 matches the sequence's shape, the entry field accepts it.

Tips & Common Mistakes

• Look at how each result builds on the previous one by adding a digit at the front, not by calculating a product or sum.
• Keep the trailing 5 in place; the new digit 4 goes in front of 3215, producing 43215.
• Enter 43215 and press OK; if the answer is rejected, re-check that the leading digit follows the 3 in the line above.