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Let present age of old member = x
let present age of new member = y
need to find (x-y)
let sum of present age of rest members = a

Since present average of 10 members are same as it was 4 years earlier,
y+a = (x-4)+(a-9*4)
⇒ y + a = x + a - 40
⇒ x+a -40 = y+a
⇒ x - y = 40

The new member is 40 years younger than the old member
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