Y=18
Explanation:
the initial statement is
y
∝
x
z
to convert to an equation multiply by k the constant
of variation
⇒
y
=
k
×
x
z
=
k
x
z
to find k use the given condition
y
=
18
when
x
=
15
and
z
=
5
y
=
k
x
z
⇒
k
=
y
z
x
=
18
×
5
15
=
6
equation is
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
∣
∣
∣
2
2
y
=
6
x
z
2
2
∣
∣
∣
−−−−−−−−−−−−
when
x
=
21
and
z
=
7
then
y
=
6
×
21
7
=
18
