$log_{a}b$ =$\dfrac{log\:b}{log\:a}, log_{b}c$ =$\dfrac{log\:c}{log\:b}, log_{c}a$ =$\dfrac{log\:a}{log\:c}$

$log_{a}b \times log_{b}c \times log_{c}a$ =$\dfrac{log\:b}{log\:a}\times\dfrac{log\:c}{log\:b}\times\dfrac{log\:a}{log\:c}$=$\dfrac{1}{2}\times\dfrac{1}{3}\times\dfrac{k}{5}$

=>1=$\dfrac{k}{30}$

=>k = 30