related to a logic in reports

hello experts,

i have a small issue...i have 3 numbers...suppose 10 14 and 18 and i got one more number 15 which is to be compared with the first 3 values...this 15 dont match with any of the 3 values(10,14 and 18)...in this case i got to print the nearest value i.e., 14....

plz tell me the logic how i can do that