Please answer this question I need it fast:Together, Mary Sue and Betty have 603 necklaces. Mary Sue has 115 more necklaces than Betty. How many necklaces does Betty have?

Let's call:
M = necklaces owned by Mary Sue
B = necklaces owned by Betty

You know that:
M + B = 603
M - B = 115

Therefore, when you have the sum and the difference of two numbers, in order to find the biggest number you have to sum the sum and the difference and divide it by 2, and in order to find the smallest number you have to subtract the difference from the sum and divide it by 2:

M = [(M + B) + (M - B)] ÷ 2
   = [603 + 115] ÷ 2
   = 359

B =  [(M + B) - (M - B)] ÷ 2
   = [603 - 115] ÷ 2
   = 244

Hence, Mary Sue has 359 necklaces and Betty has 244 necklaces.