Question 285841
don't know why you want to do that 
20=5f+15
subtract 15 from both sides
5=5f
1=f
but anyways 
20=5f+15
20-5f+15
-5f=-5
f=1

negative
you subtract 5f from both sides
you want 0 instead of 5f so you have to subtract
if you added 5f you get
20+5f=5f+5f+15
20+5f=10f+15