Let Y = sum registered
N = Number of notes counted
a = value of notes
S= sum in the register when counting began...
then by your question...
An electronic bank teller registered $775 after it had counted 120 notes and $975 after it had counted 160 notes.
Find a formula for the sum registered, in terms of the number of notes counted.
Was there a sum already on the register when counting began?
If so, how much?
Thanks in advance.