An easy maths question

I don't.

Angle of 0 (forwards)
Sin(0) = 0
Cos(0) = 1

therefore I'm saying x=x+(0*20) z=z+(1*20)

Angle of 90 (move right)
sin(90) = 1
cos(90) = 0

therefore x=x+(1*20) z=z+(0*20)