Currently, you have $60 and your sister has $135. you decide to save $5 of your allowance each week, while your sister decides to spend her whole allowance plus $10 each week. how long will it be before you have as much money as your sister?

It will take 5 weeks for both to have the same amount of savings.
It takes a lot longer to explain what is going on than to do the actual maths with shortcuts.

Explanation:
Let the count of weeks be
w
You start off with $60 in your savings
This is increased by $5 each week so the total in your savings at any given count of weeks count is:
$
60
+
$
5
w
Equation(1)

Your sister does not add to her savings but reduces it by $10 each week. So her savings at any given week count is:
$
135
−
$
10
w
Equation(2)

The count of weeks when both savings are equal is defined by the model:
Equation(1) = Equation(2)
60
+
5
w
=
135
−
10
w

←
in dollars
Add
10
w
to both sides
60
+
5
w
+
10
w
=
135
Subtract 60 from both sides
5
w
+
10
w
=
135
−
60
Simplify
15
w
=
75
Divide both sides by 15
Did you know you can treat units of measurement ($) the same way you do the numbers.
w
=
$
75
$
15

→
.
75
15
$
$
The $ signs cancel out so you are only left with numbers
w
=
75
15
→
75
÷
15
15
÷
15
=
5
1
=
5
w
=
5