In Tutorial 4, question 4 and 6, there are annuities where the time between each payment is more than a year. How do you work out the interest rate over a 4 year period?

For my answers, I just used a summation of the present values of all the separate cash flows which got me the right answers but is there a formula so that I can get the interest rate and use the annuity formula? Here is my attempt at Q6.

Also, when working from first principles, the geometric sum formula becomes very useful :)