$\displaystyle{3}\cdot{5}={15}\usepackage{cmbright}\Huge\$ , not the 375 it should multiply out to. Count up the number of times each prime appears in your factor tree. That number becomes the exponent of the prime. The correct answer is $\displaystyle{3}\cdot{{5}}^{{3}}\usepackage{cmbright}\Huge\$ .