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Bunuel
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The average weight of a group of 30 friends increases by 1 kg when the 13 Mar 2016, 12:40
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81% (01:48) correct 19% (01:54) wrong based on 182 sessions

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The average weight of a group of 30 friends increases by 1 kg when the weight of their football coach was added. If average weight of the group after including the weight of the football coach is 31 kg, what is the weight of their football coach?

A. 31 kg
B. 60 kg
C. 61 kg
D. 62 kg
E. 91 kg
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C
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The average weight of a group of 30 friends increases by 1 kg when the 13 Mar 2016, 15:17
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let c=coach's weight
original total weight=(30 friends)(30 kg avge)=900kg
(900+c)/(30+1)=31 kg avge
c=61 kg
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The average weight of a group of 30 friends increases by 1 kg when the 13 Mar 2016, 21:23
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Bunuel
The average weight of a group of 30 friends increases by 1 kg when the weight of their football coach was added. If average weight of the group after including the weight of the football coach is 31 kg, what is the weight of their football coach?

A. 31 kg
B. 60 kg
C. 61 kg
D. 62 kg
E. 91 kg
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Hi,
Again a Q which doesn't require too much of calculations..

But another approach-


Initial average= 30 and final=31..
so the weight of coach = final average and increase in weight of others= 31+30*(31-30)= 31+30=61
C

This approach is quite useful on many other Qs with cumbersome calculations or some trap
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The average weight of a group of 30 friends increases by 1 kg when the 13 Mar 2016, 21:44
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Bunuel
The average weight of a group of 30 friends increases by 1 kg when the weight of their football coach was added. If average weight of the group after including the weight of the football coach is 31 kg, what is the weight of their football coach?

A. 31 kg
B. 60 kg
C. 61 kg
D. 62 kg
E. 91 kg
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Weight of the coach is 31*31-30*30 = 61
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The average weight of a group of 30 friends increases by 1 kg when the 18 Dec 2016, 11:19
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Here is my approach to this one ->

sum(30)=900
sum(31)=961

Hence Coach must be 61 pounds

Hence C
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The average weight of a group of 30 friends increases by 1 kg when the 19 Dec 2016, 11:24
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Bunuel
The average weight of a group of 30 friends increases by 1 kg when the weight of their football coach was added. If average weight of the group after including the weight of the football coach is 31 kg, what is the weight of their football coach?

A. 31 kg
B. 60 kg
C. 61 kg
D. 62 kg
E. 91 kg
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31*31 * 30*(31 - 1 ) = Weight of football coach

Weight of football coach = 961 - 900

So, Weight of football coach = 61

Hence correct answer will be C. 61 kg
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The average weight of a group of 30 friends increases by 1 kg when the 03 Oct 2019, 03:44
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Y/30=30

(C+Y)/31=31

C+Y-Y=31^2-30^2=(31-30)(31+30)=1*61
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The average weight of a group of 30 friends increases by 1 kg when the 14 Nov 2025, 15:24
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chetan2u


Hi,
Again a Q which doesn't require too much of calculations..

But another approach-


Initial average= 30 and final=31..
so the weight of coach = final average and increase in weight of others= 31+30*(31-30)= 31+30=61
C

This approach is quite useful on many other Qs with cumbersome calculations or some trap
Show more
Hi, what's the underlying math behind this shortcut? I'm not sure how this was derived.
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The average weight of a group of 30 friends increases by 1 kg when the 15 Nov 2025, 01:24
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Z3R0TR0N

Hi, what's the underlying math behind this shortcut? I'm not sure how this was derived.
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Coach’s weight = new total weight - old total weight =
= 31 * 31 - 30 * 30

Now that difference can be rewritten as:
= (31 - 30) * 31 + (31 - 30) * 30
= 31 + 30 * (31 - 30)

So that “shortcut” 31 + 30 * (31 - 30) is just a rearranged form of 31 * 31 - 30 * 30, which is new total minus old total.
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