The Himalayas are still rising by more than 1 cm per year as India continues to move northwards into Asia, which explains the occurrence of shallow focus earthquakes in the region today. However the forces of weathering and erosion are lowering the Himalayas at about the same rate.
we can tell that the himalayas are still rising because of the movement of india northwards into asia,so the more india moves to the north the more the himalayas rise by more than 1cm.
So the total materials needed would be 70 yds. The only part you’re missing is side b that can be resolved by the Pythagoras Theorem that is showed in the picture below. If you need any explanations or have questions about it message me :)