One method to develop a plan for a proof is to work “backwards” by starting with the conclusion
2 answers:
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From the information obtained from the question, two equations can be created:
Let x and z be the two numbers (parts)
. . . . (1)
. . . . (2)
By transposing (2), make 'z' the subject of the equation
. . . . (3)
By substituting (3) into equation (1) to find a value for x
⇒
∴ either
OR 
Thus x = 5 or x = 10
By substituting the values of x into (2) to find z
z + (5) = 15 OR z + (10) = 15
⇒ z = 10 OR z = 5
So, the two numbers or two parts into which fifteen is divided to yield the desired results are 5 and 10.
Let x and z be the two numbers (parts)
By transposing (2), make 'z' the subject of the equation
By substituting (3) into equation (1) to find a value for x
⇒
∴ either
Thus x = 5 or x = 10
By substituting the values of x into (2) to find z
z + (5) = 15 OR z + (10) = 15
⇒ z = 10 OR z = 5
So, the two numbers or two parts into which fifteen is divided to yield the desired results are 5 and 10.