The gardeners at Middleton Place Gardens want to plant a total of 45 white and pink hydrangeas in one flower bed. In another flower bed, they want 120 hydrangeas. In this bed, they want 2 times the number of white hydrangeas and 3 times the number of pink hydrangeas as in the first bed. Use a system of equations to find how many pink and how many white hydrangeas the gardeners should buy altogether.

To approach this problem, let's define some variables to represent the number of hydrangeas we need. Let:

- x be the number of white hydrangeas in the first flower bed.
- y be the number of pink hydrangeas in the first flower bed.
- w be the number of white hydrangeas in the second flower bed.
- p be the number of pink hydrangeas in the second flower bed.

From the problem statement, we can determine the following equations:

In the first flower bed:
1) x + y = 45 (equation representing the total number of hydrangeas needed)

In the second flower bed:
2) w + p = 120 (equation representing the total number of hydrangeas needed)
3) w = 2x (equation representing the number of white hydrangeas being twice the number in the first bed)
4) p = 3y (equation representing the number of pink hydrangeas being thrice the number in the first bed)

Now, let's solve this system of equations:

From equation 3), we substitute w = 2x into equation 2):
2x + p = 120 (substituting w = 2x)

From equation 4), we substitute p = 3y into the above equation:
2x + 3y = 120 (substituting p = 3y)

We now have a system of two equations with two variables:
1) x + y = 45
2) 2x + 3y = 120

There are several methods to solve this system of equations. One option is to use substitution or elimination. Let's use the elimination method:

Multiply equation 1) by 2:
2(x + y) = 2(45)
2x + 2y = 90

Now, subtract equation 2) from the above equation:
(2x + 2y) - (2x + 3y) = 90 - 120
2x + 2y - 2x - 3y = -30
2y - 3y = -30
-y = -30

Multiply the entire equation by -1 to isolate y:
y = 30

Now, substitute the value of y into equation 1) to solve for x:
x + 30 = 45
x = 45 - 30
x = 15

So, the solution to the system of equations is x = 15 and y = 30.

Therefore, the gardeners should buy 15 white hydrangeas and 30 pink hydrangeas for a total of 45 hydrangeas in the first flower bed. In the second flower bed, they should buy 30 white hydrangeas and 90 pink hydrangeas for a total of 120 hydrangeas.