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.