Let 
represent Mandy's age and 
represent Sandy's.
From the first statement we have:
From the second statement we have:
So we need to solve this system of equations:
Express from first equation equation:
Replace found value in second equation and solve the resulting equation:
Replace found value in first equation and solve the resulting equation:
So, Mandy is 16 years old and Sandy is 40 years old. The difference between their ages is 40 - 16 = 24 years.
From the first statement we have:
From the second statement we have:
So we need to solve this system of equations:
Express
Replace found value in second equation and solve the resulting equation:
Replace found value in first equation and solve the resulting equation:
So, Mandy is 16 years old and Sandy is 40 years old. The difference between their ages is 40 - 16 = 24 years.