The 8 corners of a cube are joined in pairs by straight segments. How many of these segments are neither edges nor diagonals of a face?
All segments: 8C2 = 28
Edges 12, face diagonals 6 × 2 = 12
28 − 24 = 4 (the space diagonals)
Answer: 4
Permutations and Combinations · Exercise 4 of 5
65 moderate and difficult permutations and combinations questions, with points on a circle to read, in the style of IBPS / SBI PO and Clerk, RRB NTPC and Group D, UPSC CSAT, state PSC and IT campus placement tests.
The 8 corners of a cube are joined in pairs by straight segments. How many of these segments are neither edges nor diagonals of a face?
All segments: 8C2 = 28
Edges 12, face diagonals 6 × 2 = 12
28 − 24 = 4 (the space diagonals)
Answer: 4
Eight points are marked at equal spacing on a circle.
How many chords can be drawn joining pairs of these points?
8C2 = 28
Trap: 20 leaves out the 8 chords between neighbours.
Answer: 28
Eight points are marked at equal spacing on a circle.
How many quadrilaterals have all four corners among these points?
No three points on a circle are collinear
8C4 = 70
Trap: 1,680 counts order.
Answer: 70
There are 4 routes from A to B, 3 from B to C, and 2 direct routes from A to C. In how many ways can one travel from A to C?
Via B: 4 × 3 = 12
Direct: 2
12 + 2 = 14
Trap: 24 multiplies the direct routes in.
Answer: 14
In how many ways can the digits 1 to 9 be arranged so that the odd digits occupy the odd positions?
Odd digits 1, 3, 5, 7, 9 in positions 1, 3, 5, 7, 9: 5! = 120
Even digits in positions 2, 4, 6, 8: 4! = 24
120 × 24 = 2,880
Answer: 2,880
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