Len invests $5200 at 3% simple interest, while his friend dave invests $3600 at 5% simple interest. how long will it take for dave's investment to be worth more than leni's?
can someone provide the formula to solve the above question? thx!

Simple interest
I=PRT
Interest=Principal times Rate times Time
principal=amount invested
rate=rate in decimal
t=time in years
so
they take the same time
investment will be origianl+3% of it each time
Len=5200+(5200 times 0.03 times t)
Dave=3600+(3600 times 0.05 times t)
so when will dave's investment be more than len's?

dave>len
3600+(3600 times 0.05 times t)>5200+(5200 times 0.03 times t)
3600+180t>5200+156t
minus 156t both sides
3600+24t>5200
minus 3600 both sides
24t>1600
divide both sides  by 24
t>66.6666666666666666666
so after 67 years