If $55 is put in an account that gets 6% and i add $20 at the end of eqch year how much will i have at the end of 10 years?

You would have 513 in 10 years if you add 20 dollars at the end of each year. 

So here it is in math form (plug into calculator
1.06 means add 6% of whatever you have
so

55*1.06+20= first year
so what you do is just keep multiplying this quantity by 1.06 then adding 20 then multiplying, etc

the long form would be
[([([([([55*1.06+20]*1.06+20)*1.06+20]*1.06+20)*1.06+20]*1.06+20)*1.06+20]*1.06+20)*1.06+20]*1.06+20=362.1125
round
$362.11 after 10 years

A= b/2 because u simplying both sides of equation