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# Half of a certain radioactive substance decays every 5 hours. If curre

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Math Expert
Joined: 02 Sep 2009
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Half of a certain radioactive substance decays every 5 hours. If curre  [#permalink]

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05 Apr 2017, 04:00
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Difficulty:

15% (low)

Question Stats:

88% (01:19) correct 12% (01:40) wrong based on 46 sessions

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Half of a certain radioactive substance decays every 5 hours. If currently there are 40 grams of the substance in the lab, and 15 hours later 15 grams are added, how many hours will it take for the substance to decay to 5 grams?

A. 20
B. 25
C. 30
D. 35
E. 50

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Manager
Joined: 03 Jan 2016
Posts: 57
Location: India
WE: Engineering (Energy and Utilities)
Re: Half of a certain radioactive substance decays every 5 hours. If curre  [#permalink]

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05 Apr 2017, 04:24
Starting Time - 40 Grams

+ 5 Hrs - 20 g
+ 10 Hrs - 10 g
+ 15 Hrs - 5 g + 15 g (added) = 20 g
+20 Hrs - 10 g
+ 25 Hrs - 5 g.

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Narayana raju
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Posts: 523
Re: Half of a certain radioactive substance decays every 5 hours. If curre  [#permalink]

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05 Apr 2017, 08:06
after 15 hrs,, its 5 gm present,,,15gm more is added,,,
so, to decay to 5 gm, 10 more hrs needed
total 25 hrs,,
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Re: Half of a certain radioactive substance decays every 5 hours. If curre  [#permalink]

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11 Apr 2017, 11:46
1
Bunuel wrote:
Half of a certain radioactive substance decays every 5 hours. If currently there are 40 grams of the substance in the lab, and 15 hours later 15 grams are added, how many hours will it take for the substance to decay to 5 grams?

A. 20
B. 25
C. 30
D. 35
E. 50

Since there are 40 grams of the radioactive substance and half of the substance decays every 5 hours, we have:

After 5 hours: 40/2 = 20 grams

After 5 + 5 = 10 hours: 20/2 = 10 grams

After 10 + 5 = 15 hours: 10/2 = 5 grams

However, even though we have 5 grams left at this point, we are given that 15 grams are added to the 5 grams that are left after 15 hours. So now we have 20 grams of the substance for the continuation of the decay process:

After 15 + 5 = 20 hours: 20/2 = 10 grams

After 20 + 5 = 25 hours: 10/2 = 5 grams

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Posts: 248
Re: Half of a certain radioactive substance decays every 5 hours. If curre  [#permalink]

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11 Apr 2017, 17:05
40 - 20 = 20 in 5 hours
20 - 10 = 10 in 10 hours
10 - 5 = 5 in 15 hours
15 grams were added after 15 hours
20 - 10 = 10 in 20 hours
10 - 5 = 5 in 25 hours

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Re: Half of a certain radioactive substance decays every 5 hours. If curre  [#permalink]

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17 Apr 2017, 03:07
Bunuel wrote:
Half of a certain radioactive substance decays every 5 hours. If currently there are 40 grams of the substance in the lab, and 15 hours later 15 grams are added, how many hours will it take for the substance to decay to 5 grams?

A. 20
B. 25
C. 30
D. 35
E. 50

Initial-40 gm(half decay after every 5 hour)
5 hour=20
10 hour=10
15 hour=5(but here 15 gm added so)-20 gm
20 hour=10 gm
25 hour=5 gm
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Re: Half of a certain radioactive substance decays every 5 hours. If curre &nbs [#permalink] 17 Apr 2017, 03:07
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