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# When a ball is thrown directly upwards, it is at a height of h feet,..

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e-GMAT Representative
Joined: 04 Jan 2015
Posts: 2943
When a ball is thrown directly upwards, it is at a height of h feet,..  [#permalink]

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21 Nov 2018, 05:04
00:00

Difficulty:

35% (medium)

Question Stats:

71% (01:39) correct 29% (01:59) wrong based on 96 sessions

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When a ball is thrown directly upwards, it is at a height of h feet, above the ground, after t seconds. If h = $$800 – (t - 5)^4$$ feet, then at what height will the ball be, before 3 seconds to reach the maximum height?

A. 81
B. 175
C. 719
D. 784
E. 799

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Joined: 18 Jul 2018
Posts: 981
Location: India
Concentration: Finance, Marketing
WE: Engineering (Energy and Utilities)
Re: When a ball is thrown directly upwards, it is at a height of h feet,..  [#permalink]

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21 Nov 2018, 05:11
Height, $$h = 800 - (t-5)^4$$
Height will be maximum when t is 5 seconds.

Height, h before 3 seconds to maximum height is when t is 2 seconds.

$$h = 800 - (2-5)^4$$
$$h = 800 - (-3)^4$$
$$h = 800 - 81$$
$$h = 719$$

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e-GMAT Representative
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When a ball is thrown directly upwards, it is at a height of h feet,..  [#permalink]

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23 Nov 2018, 07:19

Solution

Given:
• $$h = 800 – (t - 5)^4$$ feet, where h = height above the ground in feet

To find:
• The value of “h” when the ball is 3 seconds before it reaches the maximum height

Approach and Working:
• $$h = 800 – (t - 5)^4$$
o The maximum value of h = 800, since, the minimum value of $$(t - 5)^4 = 0$$
o Thus, at t = 5 sec, the ball reaches the maximum height

• The value of t, three seconds before this will be 5 – 3 = 2 seconds
o Implies, $$h = 800 – (2 - 5)^4 = 800 – 81 = 719$$ feet

Hence the correct answer is Option C.

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When a ball is thrown directly upwards, it is at a height of h feet,..   [#permalink] 23 Nov 2018, 07:19
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