finances $-$ hire purchase grade $9$

the hire purchase price of a refrigerator is R$6500.$ the deposit of R$500$ is made and the remainder is paid in equal monthly payments of R$250.$

a$.$ calculate the number of monthly payments that must be made.

b$.$ if the cash price is R$4000,$ express as a percentage of the cash price, the extra cost of buying on hire purchase.

c$.$ what is the interest rate $($simple interest$)$ charged on this transaction.