Thinking Skills
A 5 × 5 grid of equal cells is shaded according to this rule: a cell is shaded when row × column (counting fr…
Question 1 · Multiple choice
Question
A 5 × 5 grid of equal cells is shaded according to this rule: a cell is shaded when row × column (counting from 1 in the top-left) is even; otherwise it is unshaded.
How many shaded cells are there in total?
Options
- A.16
- B.14
- C.18
- D.12
Correct answer
Explanation
Step 1 — Understand the shading rule.
A cell is shaded when row × column = even number.
A product is even whenever AT LEAST ONE of the factors is even. The ONLY way to get an odd product is if BOTH the row AND column numbers are odd.
Step 2 — Find the unshaded cells (both odd).
In a 5×5 grid:
- Odd rows: 1, 3, 5 → 3 rows
- Odd columns: 1, 3, 5 → 3 columns
Unshaded cells = 3 × 3 = 9 cells (every combination of odd row and odd column)
Step 3 — Visualise the grid (■ = shaded, □ = unshaded).
Col1 Col2 Col3 Col4 Col5
Row1: □ ■ □ ■ □
Row2: ■ ■ ■ ■ ■
Row3: □ ■ □ ■ □
Row4: ■ ■ ■ ■ ■
Row5: □ ■ □ ■ □
The 9 unshaded squares (□) appear at the odd-odd intersections. Everything else is shaded.
Step 4 — Count shaded cells.
Total cells = 5 × 5 = 25
Shaded cells = 25 − 9 = 16
Answer: 16



