Help me pls ASAP !!

If it takes 700 million years for half of your the uranium in a rock to decay into lead how old is a fossil that is:

a.) 50% uranium and 50% lead

b.) 25% uranium and 75% lead

The answer is a).I think so...Tell me if I'm wrong

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.

i think it’s c but i might be wrong. if i am let me know! have a good rest of your day as well!