Answer:
Let Kirsten and Ryan start their work on 1 st of July.
Kirsten working days are 1,4,7,10,13,16,19,22,25,28,31,...
Ryan working days are 1,5,9,13,17,21,25,........
Now suppose 1 st of July is Monday,so coming Saturday will be on 6,13,19,25,31,....
Yes, Kirsten and Ryan will work on same day again i.e on 13, 25 in the month of July.
Yes ,they have to do the two jobs on one day also i.e on day which Saturday falls i.e on 13, and 25 of july.