Sophie and simon are peeling a pile of potatoes for lunch in the cafeteria. sophie can peel all the potatoes by herself in 45 minutes, while it would take simon 30 minutes to do the job working alone. if sophie and simon work together to peel the potatoes, how long will it take them? 15 minutes 18 minutes 38 minutes 75 minutes

Short 18 minutes
Remark
The answer to this problem is less than the smallest time of the two people working together. that fact lets out C and D (38 minutes and 75 minutes). Now you have to choose 15 minutes and 18 minutes. There's a method. No guessing needed.

Givens
Let the time for Sophie = S
Let the time for Simon = M
Let the job to completion = 1 
S = 45 minutes
M = 30 minutes

Step One
Convert minutes to hours.
45 minutes = 45 / 60 = 3/4 hour = 0.75 hour
30 minutes = 30 / 60 = 1/2 hour = 0.50 hour

Step Two
Set up the Equation
The formula is a form of job / hour.
Let the time = t that they both have to work

job = 1 in these problems.
1/S + 1/M = 1/t
1/0.75 + 1/0.5 = 1/t

Solve
1 ÷ 0.75 = 1.33333
1 ÷ 0.5 = 2

1.3333 + 2 = 3.33333 
3.3333 = 1 / t                    Multlply both sides by t
3.3333*t = 1
t = 1 / 3.333333333
t = 0.3 of an hour

1 hour = 60 minutes
0.3 hours = x              Cross Multiply
x = 60 * 0.3
x = 18 minutes

Answer working together it took them 18 minutes  <<<<<