:11. Solution. 105 ounces is 1 SD above average. Hence the
average age at which such babies first sleep through the night is
0.6 x 1 = 0.6 SD's below average (below because r = -0.6). This
is 50 - (0.6 x 10) = 50 - 6 = 44 days. The SD of these ages is
given by the rms error. Since the square root of 1 - (0.6)^2 is
0.8, the rms error is 0.8 x 10 = 8 days. Since (50 - 44)/8 = 6/8
= 3/4 = .75, the perentage of such babies who first sleep through
the night at an age no older than 50 days is equal to the area
under the normal curve to the left of .75. The area under the
normal curve between -.75 and .75 is about 54%, so the desired
percentage is about equal to 54 + (46/2) = 54 + 23 = 77%. The
closest choice is 75%.