haw do I fined out the monthly payment's. For a home I want to by

You  can use this formula

for instance, if the car costs $20 000   

The interest is 18% p.a (reducable) and payments are made monthly for 5 years

n = Number of months = 5*12 = 60

i = interest per month = 0.18/12 = 0.015

PV = present value (of the loan) = $20000

I will enter it inot the web2 calc like this

20000=C*((1-1.015^-60)/0.015)

and this is the output

$${\mathtt{20\,000}} = {\mathtt{C}}{\mathtt{\,\times\,}}\left({\frac{\left({\mathtt{1}}{\mathtt{\,-\,}}{{\mathtt{1.015}}}^{-{\mathtt{60}}}\right)}{{\mathtt{0.015}}}}\right) \Rightarrow {\mathtt{c}} = {\frac{{\mathtt{115\,119\,700\,000}}}{{\mathtt{226\,672\,237}}}} \Rightarrow {\mathtt{c}} = {\mathtt{507.868\: \!548\: \!542\: \!184\: \!281\: \!7}}$$

So the payments will be   $507.87 per month for 5 years