105 minutes
"Stan and Hilda can mow the lawn in 35 min"
35 = 1/(S + H)
"Hilda works twice as fast as Stan"
H = 2S
"how long would it take Stan to mow the lawn alone?"
T = 1/S
First, let's determine what S is, so
35 = 1/(S + H)
Substitute 2S for H
35 = 1/(S + 2S)
Solve for S
35 = 1/(3S)
35(3S) = 1)
105S = 1
S = 1/105
Now solve T = 1/S with S = 1/105
T = 1/S
T = 1/(1/105)
T = 1/1 / 1/105
T = 1/1 * 105/1
T = 105
So it will take Stan 105 minutes to mow the lawn alone.
