Worded algebra question

Question

Ryan can mow a lawn in 1 hour and his son Malik can mow the same lawn in 50 minutes. One day Malik started mowing by himself in 30 minutes. Then Ryan joined him and they finished the lawn. How long did it take them to finish the lawn after Ryan joined to help. 

Answer 1

Let T be the time (minutes) worked by Ryan, then T+30 is the time worked by Malik

 

(T+30)/50 is the portion Malik mowed and T/60 is Ryan's portion.

Together they did all of the job, that is

 

[(T+30)/50] + (T/60) = 1

Multiply through by 300

6(T+30) + 5T = 300

11T + 180 = 300

11T = 120

 

T = 120/11, i.e. a little less than 11 minutes