One fourth means (one half) of (one half) .
So the 18 months must be two half-lifes.
The half-life of this particular isotope is 9 months.
The best way for adolescent to build and protect his or her bones is to ingest food products that are rich in calcium for it is known to make bones stronger. Another way is to be able to do exercise as a daily routine so that the bones are exercised every day to prevent bone disease, this will stenghten the bones and make them more stronger.
