A particle $\mathrm{A}$ is projected from the ground with an initial velocity of $10\mathrm{ }\mathrm{m} {\mathrm{s}}^{-1}$ at an angle $60°$ with horizontal. From what height (in $\text{m}$) directly above the point of projection of the particle $\mathrm{A}$, should another particle $\mathrm{B}$ be projected horizontally with velocity $5\mathrm{ }\mathrm{m} {\mathrm{s}}^{-1}$ so that both the particles collide right before they strike the ground? [$\mathrm{g}=10 \mathrm{m} {\mathrm{s}}^{-2}$]

Hence, the correct answer is 15.