What is the minimum number of cuts required to divide a cuboid into 8 equal cuboids?
- (a)2
- (b)3
- (c)5
- (d)4
Answer
Why
Correct — B.
One plane cut splits each piece into at most two, so each cut at most doubles the count.
After 1 cut: at most 2 pieces
After 2 cuts: at most 4 pieces
After 3 cuts: at most 8 pieces
Three cuts through the middle, one parallel to each pair of faces, give 2 × 2 × 2 = 8 equal cuboids → option (b)
Why the others are wrong
- (a)2 — 2 cuts make at most 4 pieces, since each cut at most doubles the count. Two cuts at right angles give 2 × 2 × 1 = 4 cuboids, half of what is asked.
- (c)5 — 5 cuts is more than needed. Three cuts already give 8 equal cuboids, and the question asks for the minimum.
- (d)4 — 4 cuts can give 8 equal cuboids (3 parallel cuts make 4 slabs, one cut across halves each: 4 × 2 = 8), but 3 cuts do it with one fewer.
Concept
With cuts that run straight through the whole block, a, b and c cuts parallel to the three pairs of faces give (a + 1)(b + 1)(c + 1) pieces. Spread the cuts over all three directions to use the fewest.
For 8 pieces: one cut each way gives 2 × 2 × 2 = 8 with 3 cuts. All cuts in one direction need 7, and 3 + 1 needs 4.
The doubling bound is why nothing beats 3: even if pieces are stacked before a cut, each piece still splits into at most two. For 8 pieces the 1 + 1 + 1 grid meets that bound exactly.
Key facts
- One plane cut splits each piece into at most two, so n cuts give at most 2ⁿ pieces.
- a, b and c cuts in the three directions give (a + 1)(b + 1)(c + 1) pieces.
- 8 equal cuboids need 3 cuts, one parallel to each pair of faces through the middle.
Study next
Common traps
- Choosing 4 because 3 parallel cuts plus 1 across make 8: spreading the cuts over three directions saves one.
- Carrying the doubling idea to the 64-cube cake: stacking could reach 64 in 6 cuts, but its key is 9 (3 in each direction). That key assumes the pieces stay in place, which the stem never states.
17 Sep 2025, 16:00, Quant Q.12 asks the same for a cubical cake cut into 64 identical cubes, keyed 9 (option b): with the pieces left in place, 3 cuts in each direction give 4 × 4 × 4 = 64.
Related PYQs
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