An analysis of a sample rock from layer H indicates an equal proportion of parent isotope and

Question:

An analysis of a sample rock from layer H indicates an equal proportion of parent isotope and daughter produced from the parent. Fifty percent of the original parent has decayed to the daughter of the product. The half-life of the parent is known to be 425 million years. The analysis of a sample of rock from layer I indicate its age to be 400 million years.
a. How many half-lives of the parent isotope have elapsed since rock H formed?
b. Is the age of fault M older or younger than 400 million years?
c. What is the approximate maximum numerical age of the igneous intrusion L?
d. Complete the following general statement, first choose the correct option, then fill in the blank for the numerical ages of rock layers G, F, and E?
All of the rock layers are (younger, older) than ( ) million years