Answer: Multiply the number by 0.15
Example: 100 x 0.15 = 15
By knowing [tex]p\%=p/100[/tex] we know [tex]15\%=15/100=0.15[/tex].
Now if you would like to get a part of some number [tex]q[/tex], say, some percentage like 15 just multiply [tex]q\cdot p/100[/tex].
Hope this helps.
