#### Answer

(a) Gallium-69 has the greater abundance.
(b) Gallium-69 has a percent abundance of $60.11\%$, and Gallium-71 has that value equal to $39.89\%$.

#### Work Step by Step

(a) Since the atomic weight is closer to the weight of the gallium-69 isotope, it has the greater abundance.
(b)
X: Percent abundance of gallium-69.
Y: Percent abundance of gallium-71.
$(68.9256)(X) + (70.9247)(Y) = (69.7231)$
And, X + Y must equal 1.
$X + Y = 1$ $--\gt $ $X = 1 - Y$
$(68.9256)(1 - Y) + (70.9247)(Y) = (69.7231)$
$68.9256 - 68.9256Y + 70.9247Y = 69.7231$
$1.9991Y = 69.7231 - 68.9256$
$1.9991Y = 0.7975$
$Y = \frac{0.7975}{1.9991} = 0.3989$
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Therefore:
$X = 1 - Y = 1 - 0.3989 = 0.6011$
Representing those numbers in percent values:
$X = 60.11\% $
$Y = 39.89\%$