snapping numbers question please answer

i have the need to make numbers snap to a multiple of another number

so x= whatever you want
y=?
m= multiple say 10 for ease of use

i want y to equal a multiple of m averaged from x

so if x=1 to 5 y =0 and if x=6 to 15 y=10 etc etc

so i need to be able to set x and m and find y

basic snapping rules but i have no brain

i would like a non function intensive piece of code so i dont rely on anything

if someone could be of help i would be really appreciative thankyou