So, this guide is going to be very simple. It does involve math though so bare with me.
Starting out with 0 xp:
This is going to be very simple and math is involved.
P=T/B
P= Amount of bones, T=level 99 xp, B=Bones xp
We are going to use Ourg bones so it will be easier and faster if you buy them off an admin.
So:
T= 13,041,600
B= 7,600
P=T/B
P= 13,041,600/7,600
P= 1716
1716 bones from 1-99 prayer.
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Starting out with some xp:
So if you have like... 52 prayer for example, you will have to use this equation:
P=T-X/B
P= Amount of bones to 99, T= Level 99 XP, X= Xp you already have, B=Bones xp
We already know this much:
T= 13,041,600
B= 7,600
P=13,041,600-X/7600
Let's say X= 45,600.
P=13,041,600-45,600/7600
P=12,996,000/7600
P=1710
It will take you 1710 ourg bones to get 99 prayer from starting out with 45,600 xp.
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*EDIT*
To find out how many packs that will take you to get to 99, you take the amount of bones it will take to get to 99 prayer and divide by the amount of space you get in your inventory. So if you are buying the bones from shop, you will divide the amount of bones by 27. Z=P/27. If an admin or player got you enough for 99 and you take out of the bank, then you do this: Z=P/28. Z=Packs, P=Bones to 99 prayer.
Portal 
