It is that simple. Imagine (A) is initially the YZ plane. First rotate $20^\circ$ about Y. This gives us a $20^\circ$ angle between (A) and Z. Now apply a $35^\circ$ rotation about Z. You can imagine this rotation as applying to both (A) and Z (since it doesn't change Z); and rotations preserve angles, so the angle between (A) and Z must still be $20^\circ$. At the same time, we now have an angle of $35^\circ$ between (A) and Y as desired.

It is that simple. Let $L_{XY}$ be the intersection of (A) with the XY planes, and $L_{XZ}$ with the XZ plane. Imagine (A) is initially the YZ plane. First rotate $20^\circ$ about Y. This gives us a $20^\circ$ angle between $L_{XZ}$ and Z. Now apply a $35^\circ$ rotation about Z. Notice that this fixes $L_{XY}$, which is equal to Y at this point. You can imagine this rotation as applying to $L_{XZ}$ and Z both (since it doesn't change Z); and rotations preserve angles, so the angle between $L_{XZ}$ and Z must still be $20^\circ$. At the same time, we now have an angle of $35^\circ$ between $L_{XY}$ and Y as desired.

It is that simple. Imagine (A) is initially the YZ plane. First rotate $20^\circ$ about Y. This gives us a $20^\circ$ angle between (A) and Z. Now apply a $35^\circ$ rotation about Z. You can imagine this rotation as applying to both (A) and $Z$ (since it doesn't change $Z$); and rotations preserve angles, so the angle between (A) and $Z$ must still be $20^\circ$. At the same time, we now have an angle of $35^\circ$ between (A) and Y as desired.

It is that simple. Imagine (A) is initially the YZ plane. First rotate $20^\circ$ about Y. This gives us a $20^\circ$ angle between (A) and Z. Now apply a $35^\circ$ rotation about Z. You can imagine this rotation as applying to both (A) and Z (since it doesn't change Z); and rotations preserve angles, so the angle between (A) and Z must still be $20^\circ$. At the same time, we now have an angle of $35^\circ$ between (A) and Y as desired.