Let n represent the number.

n-2.5 subtract 2.5

2(n-2.5) double the answer

2(n-2.5)+7 add 7

(2(n-2.5)+7)/2 halve the answer

(2(n-2.5)+7)/2=15 make equation.

Solve the equation:

Double both sides: 2(n-2.5)+7=30

Subtract 7 from each side: 2(n-2.5)=23

Halve each side: n-2.5=11.5

Add 2.5 to each side: n=14, the number was 14.

There’s another method you can use which you might find easier: the Undo Method:

Read your word problem backwards, reversing the instructions.

Halve the answer becomes double the answer. The answer was 15, so double it=30.

Adds 7 becomes subtracts 7=23.

Doubles her answer becomes halves her answer=11.5.

Subtract 2.5 becomes add 2.5=14, the number you started with!