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Part 1. x = 20 - y and z = 3y - 4

Part 2.  Mike's age = x = 15 years and Joe's age = z = 11 years.

Part 3. Sally's age will then be 10 years.

Step-by-step explanation:

Given that Sally's age is y years.

The sum of Sally's age and Mike's age is 20.

Joe is 4 years less than triple Sally's age.

Part 1.

Therefore, Mike's age is x = 20 - y.

And Joe's age is, z = 3y - 4.

Part 2.

If Sally's age were 5 years, the Mike's age = x = 20 - 5 = 15 years.

And Joe's age = z = 3 ×5 - 4 = 11 years.

Part 3.

If Sally and Mike has the same are i.e. x = y = 20 - y

⇒ 2y = 20

⇒ y = 10 years.

Therefore, Sally's age will then be 10 years. (Answer)