we know that
[circumference]=2*pi*r
for diameter=5 cm
r=5/2----> r=2.5 cm
[circumference]=2*pi*2.5-----> 5*pi cm
for diameter=15cm
r=15/2----> r=7.5 cm
[circumference]=2*pi*7.5-----> 15*pi cm
the answer is
the circumference will be three times higher
