Mike started a savings account by depositing $9. each month he deposits more money than the month before. at the end of 41 months, he has saved $9,389.00. how much more does he deposit each month

We have to calculate how much more money does Mike deposit each month. This can be done using the formula for the arythmetic sequence; S n = n/2 * ( a1 + an ). Here: a 1 = 9, n = 41 and a n = a 1 + ( n-1 ) d = 9 + 40 d. So : 9,389 = 41/2 * ( 9 + 9 + 40 d ); 9,389 = 41 / 2 * ( 18 + 40 d ); 9,389 = 41 * ( 9 + 20 d ) ; 9,389 = 369 + 820 d; 820 d = 9,389 - 369; 820 d = 9,020; d = 9,020 : 820; d = 11. He deposits $11 more each month.