What am I doing wrong? (SAT math)

Test Preparation SAT Preparation
1

<p>I don’t understand why I got these SAT math questions wrong.</p>

<p>1) Given function: h(t) = c - (d-4t)^2</p>

<p>At time t = 0, a ball was thrown upward from an initial height of 6 feet. Until the ball hit the ground, its height, in feet, after t seconds was given by the function h above, in which c and d are positive constants. If the ball reached its maximum height of 106 feet at time t = 2.5, what was the height, in feet, of the ball at time t = 1?</p>

<p>2) If a, b, c, and f are four nonzero numbers, then all of the following proportions are equivalent EXCEPT</p>

<p>a. a/f = b/c
b. f/c = b/a
c. c/a = f/b
d. a/c = b/f
e. af/bc = 1/1</p>

<p>I got 28.6 for 1 and E. for 2. Apparently they are wrong. Can anyone tell me the correct answers and how they got those?</p>

<p>I usually get 800s on practice, but I got these wrong on the blue book (1 is from practice test #5 and 2 is from practice test #4)</p>

2

<p>For 1: </p>

<p>At time t=0, height equals the initial height, or 6.
6 = c - d^2</p>

<p>At time t=2.5, substituting in the values gives:
106 = c - (d-10)^2</p>

<p>Subtracting the former equation from the latter gives:
100 = d^2 -(d-10)^2
100 = d^2 - d^2 + 20d - 100
100 = 20d - 100
d = 10</p>

<p>So c is equal to 106. </p>

<p>Knowing this, at time t = 1,
h = 106 - (10-4)^2
h = 106 - 36
h = 70</p>

<p>For 2: </p>

<p>This is all based off cross-multiplication.
If f/c = b/a (b), then cross-multiplication gives bc = af. This can be rearranged to give c/a = f/b (c), a/c = b/f (d), and af/bc = 1/1 (e). Therefore, choice (a) is the only one which isn’t equivalent to the others.</p>

3

<p>^Agreed. (just checked his/her work)</p>