John's hourly rate is 1/8 of the room per hour.
Rick's hourly rate is 1/2 of the room per hour.
Molli's hourly rate would be 3/8 of the room per hour.
It takes Molli 2 2/3 hours to paint the room alone.
Explanation
Since John paints the entire room (100%=1) in 8 hours, he would paint 1/8 of the room in 1 hour.
Since Rick paints the entire room (100%=1) in 2 hours, he would paint 1/2 of it in 2 hour.
We do not yet know Molli's rate, so we will call it x.
We know that 1/8+1/2+x = 1 hour.
We will use 8 as a common denominator:
1/8+4/8+8x/8 = 1
Adding the numerators,
(1+4+8x)/8 = 1
Multiply both sides by 8:
1+4+8x = 8
Add like terms:
5+8x = 8
Subtract 5 from both sides:
5+8x-5=8-5
8x=3
Divide both sides by 8:
8x/8 = 3/8
x = 3/8
Molli's rate is 3/8.
Since she can paint 3/8 of the room in 1 hour, we can set up the equation
3/8x = 1
Divide both sides by 3/8:
3/8x ÷ 3/8 = 1 ÷ 3/8
x = 1/1 ÷ 3/8 = 1/1 × 8/3 = 8/3
x = 2 2/3
This is the amount of time it takes Molli to paint the entire room.
