(emergency) maths equation form 1

Let the speed of mary is x piece per day, the speed of ann is y piece per day. Total speed is (x+y) piece per day
Then we have
3(x+y)=1 (total work 1 piece)
On the other hand
Mary alone will finish it in 5days. So
5x=1
Then
3(x+y)=5x
3y=2x
y=(2/3)x
From 5x=1, x=0.2. y is 2/15
The speed of ann is 2/15 piece per day
The days Ann need to finish the work by herself
=1/(2/15)
=15/2
=7.5 days