Answer:
The amount that William has at the beginning is $37.50 and the amount that Gracie has at the beginning is $52.50
Explanation:
Let
x ---> amount that William has at the beginning
y ---> amount that Gracie has at the beginning
we know that
William says to Gracie, "If you give me $7.50,we will have and equal amount of money
so

----> equation A
"Grace responds, if you give me $ 7.50 instead, I will have twice as much money as you


----> equation B
substitute equation A in equation B

solve for x

Find the value of y

therefore
The amount that William has at the beginning is $37.50 and the amount that Gracie has at the beginning is $52.50