Here is the work hope this helps!
20
Step-by-step explanation:
Step 1:
Let the number be 50 and to find 40% of 50 is given interms of expression as follows
To express the percentage the following strategy is used
Eg: 40% = 
∴ To express 40% of 50 is

Step 2:
On simplification the above expression we could get

= 20
Let her sisters age be x.
=>sister .....x
Sarah.
twice x=2x. then less than 5
=> (2x-5 )
if Sarah = 15 years
by substation, 2x-5=15
The answer is whole
Hope this helps