A "grad" measurement is one done with the Gradian scale, where 400 Gradians = a full circle. Remember that 360 degrees also equals a full circle. That means the conversion is G/400 = D/360
You are told that G = 15 + D (or equivalently, D = G - 15).
So substitute for one of the variables in the conversion equation and solve for the remaining measurement.
Hope this helps.
Stephen La Rocque>