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a,b,c,d  and e are five consecutive numbers in increasing order of size. Deleting one number from the set decreased the sum of the remaining numbers in the set by 20%.

Which one of the following numbers was deleted?

  • A.a
  • B.b
  • C.c
  • D.d
  • E.e

Answer: C

Five consecutive numbers are symmetric about the middle one, so their total is five times that middle value. Writing the set as \( c-2,\, c-1,\, c,\, c+1,\, c+2 \):

\[ S = 5c \]

Removing one member cuts the total by 20 per cent, so the number removed is

\[ 0.20 \times 5c = c \]

which is the middle number, c.

The result holds whatever the five numbers are, since the arithmetic never uses their actual values - only that they are consecutive and so centred on \( c \).

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