John wishes to accumulate $75,000 in a fund at the end of 15 years (t = 15). He will deposit $1200 + t*Z at the end of each year for the first 10 years (t=1,2,3,...,10) and $1000 at the end of the last five years (t=11,12,...,15). John's fund will earn interest at an annual effective rate of 7%. Determine Z. NO EXCEL. a. = $500 but = $550 but = $600 but= $650