find the circumference of a circle whose radius is 15.4cm 

[latex]r=15,4\ cm\\C=2\cdot\ \pi\ \cdot\ r\\C=2\cdot\ \pi\ \cdot\ 15,4\ cm\\C=30,8\ \pi\ cm[/latex] [latex] \pi\approx\ 3,14[/latex] [latex]C=30,8\ \cdot\ 3,14\ cm\\C=96,712\ cm[/latex]

π = 3,14 c = 2.π.r c = 2.3,14.15,4 c ≈ 96,7