Those equations are all correct, if you are getting funny answers then it is due to some error in your working rather than the equations themselves. If you want to post (or PM) the examples you are having trouble with and your working then I'm sure we will be able to help you identify the areas where you are going wrong.
Are you accounting for the fact that your units of measurement might be different in the different quantities you are using? For instance - speed is usually measured in terms of miles per hour,but the times you are using may well be in minutes so you will need to convert them to hours (e.g. 10mins = 1/6 hour, 15mins = 1/4 hour, 12mins = 1/5 hour etc.) before putting the figures into the equations.