If a person starts investing $100 per month starting at age 21, and that money earns a 7% return every year, how much will this person have when turning 70 years old? For ease of calculation, assume starting balance of $0 and annual contributions of $1,200 (12*$100

The formula of the future value of an annuity ordinary is
Fv=pmt [((1+r)^(n)-1)÷r]
Fv future value?
PMT yearly payment 1200
R interest rate 0.07
N time 49 years (70-21)

Fv=1,200×(((1+0.07)^(49)−1)÷(0.07))
Fv=454,798.80

Hope it helps!