What is the phase difference between points P and R in Figure 4?

[PhysicsAShelp]

Phase Difference - Waves! Help! URGENT

Homework Statement

Firstly, can I add that this question is second part of a question , the first part asked me to find the amplitude and wavelength of a wave from a picture, I got these to be a) Amplitude = 8mm - b) Wavelength = 47mm <<<<These two are 100% correct values, I checked the answers at the back of the book.

So, the question I'm stuck on is "For the waves in figure 4: - determine the phase difference between P and R." I've measured this and the distance between the two is 100% 24.5mm. From this I use the equation:
the phase difference in radians = 2 x Pi x d/\lambda

Homework Equations

the phase difference in radians = 2 x Pi x d/\lambda

The Attempt at a Solution

From the phase difference in radians = 2 x Pi x d/\lambda
I do, 2 x Pi x 0.0245/0.047 = 3.2757... This gives my answer in rads, but, from here one, do you convert it too degrees, secondly is this correct.
The correct answer at the back of the book is 180degrees.

 

[payumooli]

i can only tell you that the last arithmetic is correct

if you convert it into degrees (3.27*180/pi) you get 187 deg which is 7 deg off by your book result

 

[PhysicsAShelp]

Hmmm, I wonder if maybe you do it to the nearest 90? Because on another question I get 268 the answer is 270 though...?

EDIT:: For the C1 exam do you guys reckon if it asked for the phase difference in degrees I'd round it to the 180,270 etc or just the normal 3sigfig?