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!