Hi,

Create your reference in a table and use vlookup() function
a1 = 1 a2 = 2 a3 = 3
b1 = 25 b2 = 50 b3 = 150

then if you input 1 in cell C1, you can have in cell D1 :
=vlookup(c1,a1:b3,2)

HTH
Cheers
Carim