Question

they've invested to earn $20 in a savings account that offer 3% return on the investment the value of david's investment will be at least $400 after a period of

Answer 1

I=PRT
P=present amount
R=rate
T=time
basically
20+interest is more than or equal to 400
20+I>400
minus 20 both sides
I>380

solve
P=20
R=3%=0.03
T=?
I=PRT
380<I=PRT
380<20(0.03)(T)
380<0.6T
divide both sides by 0.6
633.33333<T
633.33 years at least