You must see the mass of Gallium in the periodic table. The mass present there is calculated by a weighted average of the mass of the isotopes, where the weights are the abundances of each isotope. Let X the isotopic abundance of Ga⁷¹. Since there are only Ga⁶⁹ and Ga⁷¹, the isotopic abundance of Ga⁶⁹ is 1-X. Then:
\(m(Ga)_{periodic~table}=(1-X)\cdot m(Ga^{69})+X\cdot m(Ga^{71})\\\\69.7=(1-X)\cdot69+X\cdot71\\\\
69.723=69-69X+71X\\\\
2X=0.723\\\\
X=0.3615\\\\
\boxed{X=36.15\%}\)
I don't know the options, but probably 40% is the closest to 36.15%.
