Reaction between Zinc and dilute sulfuric acid: a student measures the time taken for 25cm3 of H2 gas to be given off. zinc granules:225sec, zinc pellet:114sec. Mean rate of reaction=? For each?

You just divide 25 cm^3 by 225 sec for the zinc granules, and for the zinc pellets 25 cm^3 by 114 sec. This will give you a reaction rate in cm^3 per SECOND (cm^3/s). If you want to do it per MINUTE then calculate how many minutes make up 225 seconds and 114 seconds, given that there are 60 seconds in a minute.