Let the number of hour it took them to have completed the same number of problems be x, then
15 + 10x = 9 + 12x
12x - 10x = 15 - 9
2x = 6
x = 6/2 = 3
After 3 hours, they must have completed the same number of homeworks.
Answer:
The formula for calculating the width of the window is

Step-by-step explanation:
<u><em>The question in English is</em></u>
A rectangular window is l meters wide and h meters high, with a perimeter of 6 meters and an area of 2m². What is the formula for calculating the width of the window?
we know that
The perimeter of the window is equal to

we have

so

simplify
isolate the variable l
----> equation A
The area of the window is equal to

we have

so
----> equation B
substitute equation A in equation B
solve for w

The formula to solve a quadratic equation of the form
is equal to
in this problem we have

so
substitute in the formula
---> formula for calculating the width of the window
a) 9 toilet rolls cost $ 4.23
1 toilet roll costs $ x
9x = 4.23 * 1
x= 4.23 / 9
x = 0.47
So in first case one toilet roll costs $ 0.47.
b) 4 toilet rolls cost $1.96
1 toilet roll costs $ x
4x = 1.96 * 1
x = 1.96 / 4
x = 0.49
So in second case one toilet roll costs $ 0.49.
0.47 < 0.49
The toilet roll in first case is cheaper than the toilet roll in second case.
Hope this heps!