A sector of a circle of radius 15 cm with a central angle of 216° is rolled up, edge to edge, to form an open cone. What is the radius of the cone's base, in cm?

Rolling the sector makes its ARC become the circumference of the cone's base, while the sector's radius becomes the cone's slant height. Arc = (216/360) $\times$ 2π $x15=0.6\times 30$π = 18π cm, so 2π$r=18$π and $r=9$ cm. The trap 15 quotes the slant height as the base radius; 18 stops at arc/π and forgets the 2 in 2π$r$; 4.5 halves the answer once more.