Absorbance =

100÷%T
A=

2.105 <=>
<=> A=0.3233
To 47.5%T corresponds 0.3233 absorbance.
Hi, there! (:
Well for starters - you shouldn't take your friend's medication because it can internally harm you. Chances are if it's prescribed by a doctor for someone other than you it's not made for your body which can lead to a very dangerous reaction on your end.
Another reason is that it's not actually legal.
But lastly, it can lead to a road of addiction, or even you going and trying to experiment further, and it could lead to long term damage.
I hope all is well, and you pass! Good luck, rockstar! (: If you have any follow up questions, or even a comment on what i've wrote - let me know! (:
Target heart rate is defined as the minimum number of heartbeats in a given amount of time in order to reach the level of exertion necessary for cardiovascular fitness, specific to a person's age, gender, or physical fitness.