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Chu-Li has $100 in her piggy bank. How much will she have in her bank

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Chu-Li has $100 in her piggy bank. How much will she have in her bank  [#permalink]

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New post 17 Aug 2018, 12:26
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Chu-Li has $100 in her piggy bank. How much will she have in her bank 40 weeks from now if she puts $1 in the bank next week, $2 two weeks from now, $3 three weeks from now, and continues to increase the amount that she puts in by $1 each week?

A. $140
B. $800
C. $920
D. $1640
E. $1740

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Re: Chu-Li has $100 in her piggy bank. How much will she have in her bank  [#permalink]

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New post 17 Aug 2018, 12:41
Bunuel wrote:
Chu-Li has $100 in her piggy bank. How much will she have in her bank 40 weeks from now if she puts $1 in the bank next week, $2 two weeks from now, $3 three weeks from now, and continues to increase the amount that she puts in by $1 each week?

A. $140
B. $800
C. $920
D. $1640
E. $1740



100 + sum of 1st 40 terms
100 + 40(41)/2
100 + 820
920
C
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Re: Chu-Li has $100 in her piggy bank. How much will she have in her bank  [#permalink]

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New post 17 Aug 2018, 12:42
Bunuel wrote:
Chu-Li has $100 in her piggy bank. How much will she have in her bank 40 weeks from now if she puts $1 in the bank next week, $2 two weeks from now, $3 three weeks from now, and continues to increase the amount that she puts in by $1 each week?

A. $140
B. $800
C. $920
D. $1640
E. $1740


Cumulative amount of money in 40 weeks: 1+2+3+...............+40=\(\frac{40*41}{2}\)=820

Total savings=Base deposit+Additional deposit in 40 weeks=100+820=$920

Ans. (C)
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Re: Chu-Li has $100 in her piggy bank. How much will she have in her bank  [#permalink]

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New post 17 Aug 2018, 16:08
PKN wrote:
Bunuel wrote:
Chu-Li has $100 in her piggy bank. How much will she have in her bank 40 weeks from now if she puts $1 in the bank next week, $2 two weeks from now, $3 three weeks from now, and continues to increase the amount that she puts in by $1 each week?

A. $140
B. $800
C. $920
D. $1640
E. $1740


Cumulative amount of money in 40 weeks: 1+2+3+...............+40=\(\frac{40*41}{2}\)=820

Total savings=Base deposit+Additional deposit in 40 weeks=100+820=$920

Ans. (C)


Hi, what formula is this through which we get 40*41/2?

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Chu-Li has $100 in her piggy bank. How much will she have in her bank  [#permalink]

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New post 17 Aug 2018, 21:08
1
vedanths wrote:
PKN wrote:
Bunuel wrote:
Chu-Li has $100 in her piggy bank. How much will she have in her bank 40 weeks from now if she puts $1 in the bank next week, $2 two weeks from now, $3 three weeks from now, and continues to increase the amount that she puts in by $1 each week?

A. $140
B. $800
C. $920
D. $1640
E. $1740


Cumulative amount of money in 40 weeks: 1+2+3+...............+40=\(\frac{40*41}{2}\)=820

Total savings=Base deposit+Additional deposit in 40 weeks=100+820=$920

Ans. (C)


Hi, what formula is this through which we get 40*41/2?

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Hi vedanths,

The sum of first 'n' natural numbers:\(\frac{n(n+1)}{2}\)

Here we have to determine the sum of first 40 natural numbers,(1+2+...+40)
Now put n=40 in the above sum formula.We have ,sum=40*(40+1)/2.

Hope it helps.
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Re: Chu-Li has $100 in her piggy bank. How much will she have in her bank  [#permalink]

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New post 18 Aug 2018, 02:45
PKN wrote:
Hi vedanths,

The sum of first 'n' natural numbers:\(\frac{n(n+1)}{2}\)

Here we have to determine the sum of first 40 natural numbers,(1+2+...+40)
Now put n=40 in the above sum formula.We have ,sum=40*(40+1)/2.

Hope it helps.


Yes got it, thank you!
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Re: Chu-Li has $100 in her piggy bank. How much will she have in her bank  [#permalink]

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New post 20 Aug 2018, 11:05
1
Bunuel wrote:
Chu-Li has $100 in her piggy bank. How much will she have in her bank 40 weeks from now if she puts $1 in the bank next week, $2 two weeks from now, $3 three weeks from now, and continues to increase the amount that she puts in by $1 each week?

A. $140
B. $800
C. $920
D. $1640
E. $1740


Since sum = average x quantity and we have a set of evenly spaced numbers:

The average of an evenly spaced set of numbers is (smallest + greatest)/2, so average = (1 + 40)/2 = 41/2

sum = 41/2 x 40 = 41 x 20 = 820

Since she started with 100 dollars, she will have 920 dollars after 40 weeks.

Answer: C
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Re: Chu-Li has $100 in her piggy bank. How much will she have in her bank   [#permalink] 20 Aug 2018, 11:05
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