Pls help me i cant figure it out and put a good answer

Geologists know that potassium-40 decays to argon-40, with a half-life of 1.3 billion years. if an analysis of a feldspar crystal in a sample of granite shows that 75 percent of the potassium-40 atoms have decayed to form argon-40, what is the age of the granite

Describe how ocean currents can affect a moving object ❕❕❕

What is unusual about the iraq mountains ,in spite of their old age?