Solution:

Cash Price = `800`
Down Payemnt = `500`
Remaining Balance = `800  500 = 300`
The installment to be paid at the end of `6` months = `320`
`:.` the interest charged on Rs `300`, for a period of `6` months = Rs `320`  Rs `300 = 20`
If R % is the rate of interest per annum, then `(300 × R × 6) / (100 × 12) = 20`
`R = 13.3333`
Thus, the rate of interest charged under the installment paln is = `13.3333` per annum


