Answer:
an arithmetic sequence or a linear function.
Explanation:
Arithmetic:
he has $100 and ADDS $20 each month. in arithmetic sequences we have a value for a sub 1 in this case 100, then add the rate of change or in this case 20 times the months or d. Since this function follows the rules this is an arithmetic sequence.
Linear function:
y=mx+c
let y be the amount of money he has in his banking account,
let x be months.
let m be 20 because that is what he adds per month or per x.
let c be the starting value or 100.
y=20x+100