Answer:
140 days until they both get their haircut on the same day again
Step-by-step explanation:
When two events, A and B, happen every x and y days, respectively, they will happen on the same day in each lcm(x,y) days. lcm(x,y) is the lesser common multiple of x and y which is not 0.
In this question:
Betsy every 28 days.
Mary every 35 days.
Multiples of 28: {0,28,56,84,112,140,...}
Multiples of 35: {0,35,70,105,140,...}
So the lesser common multiple between 28 and 35 is 140.
So
140 days until they both get their haircut on the same day again
