The week would be week 5 and the amount would be $525
Since both terms are perfect cubes, factor using the difference of cubes formula,
a
3
−
b
3
=
(
a
−
b
)
(
a
2
+
a
b
+
b
2
)
a
3
-
b
3
=
(
a
-
b
)
(
a
2
+
a
b
+
b
2
)
where
a
=
x
2
a
=
x
2
and
b
=
y
b
=
y
.
(
x
2
−
y
)
(
(
x
2
)
2
+
x
2
y
+
y
2
)
(
x
2
-
y
)
(
(
x
2
)
2
+
x
2
y
+
y
2
)
Multiply the exponents in
(
x
2
)
2
(
x
2
)
2
.
Hello,
We know that if we make the same operation on both sides, we don't affect the equation:
For example:

Do you see? we add 2 on both sides and we don't affect the equality
We can do the same with the multiplication, for example
4=4 (i'll multiply both sides by 1/2)

Now, we do exactly the same in your excercise:
I think it is 62.8m3 but not 100% sure