We shall break the given angle into rotation and an angle
is 
Also
lies in quadrant 3.
In quadrant 3 sin is negative
So

Hence the reference angle of 7π/6 is π/6 and sin (7π/6) = -1/2.
First you have to find the angle B (check attachment)

Then you can find the last angle inside the triangle, which I'll call C

Now you can find angle A

Difference = 48-19
= 29
production decrease = 29 × 100/ 48
= 60.41%