Keliling persegi panjang = K = 60 cm

Perbandingan panjang dan lebar = $p : l$ = 3 : 2

K = $2\times\left(p+l\right)$

60 = $2\times\left(p+l\right)$

$p+l=60\div2$

$p+l=30$

Perbandingan $p\ :\ l$ = 3 : 2

Diketahui : $p\ +l=30$

Maka,

Panjang persegi panjang = $\frac{3}{3+2}\times30$

= $\frac{3}{5}\times30=18$ cm