Mathematical Reasoning
Mia makes a number pattern using these rules:
Question 1 · Multiple choice
Question
Mia makes a number pattern using these rules:
- Choose the first three numbers.
- After this, each number is the sum of the three numbers before it.
Here is Mia’s pattern with three missing numbers labelled X, Y and Z:
3, X, 1, Y, 7, Z
All the numbers are whole numbers.
Options
What number does Z represent?
- A.11
- B.13
- C.12
- D.14
- E.15
Correct answer
Explanation
The rule in words
From the fourth number onward: each number = sum of the three numbers before it.
Sequence: 3 X 1 Y 7 Z
│ │ │ │ │
└──────┴──────┘ │ │
sums to ──────► Y │
└──────┴──────┘
sums to ──────► 7
└──────┴──────
sums to ──► Z
Step 1 — Write Y using X
The three numbers before Y are 3, X, 1:
Y = 3 + X + 1 = X + 4
Step 2 — Use the number 7
The three numbers before 7 are X, 1, Y:
7 = X + 1 + Y
Substitute Y = X + 4:
7 = X + 1 + (X + 4) = 2X + 5
2X = 2 → X = 1
Step 3 — Find Y
Y = 1 + 4 = 5
Check the row so far: 3, 1, 1, 5, 7, …
- 3 + 1 + 1 = 5 ✓
- 1 + 1 + 5 = 7 ✓
Step 4 — Find Z
The three numbers before Z are 1, 5, 7:
Z = 1 + 5 + 7 = 13
Answer: 13
Repeatable checklist
-
Label the missing letters in order.
-
Write the first missing letter using the earliest "sum of three" you can make.
-
Use the next known number to make an equation and solve.
-
Work forwards to any later missing letter.
-
Check by rebuilding the next number from three before it.
Traps
- 12 — added only 7 + 5 (two numbers, not three).
- 14 — used 3 + 1 + 7 (skipped X and Y).
- 15 — arithmetic slip on 2X + 5 = 7.