At what rate of simple
interest will $5000 amount to
$6050 in 3 years, 4 months?

P (Principal) = $5000
I (Interest) = $6050 - $5000 = $1050
T (Time) = 3 1/3 years or 40 months

Formula:
I = (PRT)/100
manipulate this and you’ll get:
R = (100I)/PT

Plug in the values:
R = (100 * 1050)/(5000 * 3 1/3)
R = 105000/16666
R = 6.3

Hence, the annual interest rate is 6.3%.
However, I did not understand whether you want the annual interest rate or monthly interest rate.
If you require monthly interest rate, then just put T (Time) = 40, to signify 40 months.

Hope this helps