How many groups of 25 are there in 500

500÷25

they are 20 groups

If you divide 25 by 500 the answer is 20 therefore there are 20 groups of 25 in 500

We have to calculate a monthly annuity.

Total = Monthly Amount * [(1 + rate)^(n + 1) -1] / [rate] - Monthly Amount

where "n" is the number of compounding periods per year.
and n = 4.5*12 = 54
rate = rate/n = .055 / 12 = 0.0045833333

Total = 100 * [(1.0045833333)^55 -1] / [ 0.0045833333 ] -100
Total = 100 * 0.2859624157 /  [ 0.0045833333 ] -100
Total = 28.59624157 / [ 0.0045833333 ] -100
Total = 6,239.18 -100
Total = 6,139.18

Source:
http://1728.org/annuity3.htm  formula
http://1728.org/annuitym.htm calculator