Ugly, but effective:
if(n < 20, switch(r,1,29,2,26,3,23,4,21,5,19,6,17,7,15,8,13,9,10,10,7), if(n < 30, switch(r,1,33,2,30,3,27,4,24,5,21,6,19,7,17,8,15,9,13,10,10), if(n < 40, switch(r,1,37,2,34,3,31,4,28,5,25,6,22,7,19,8,17,9,15,10,13), switch(r,1,42,2,38,3,35,4,31,5,28,6,25,7,22,8,19,9,17,10,15))))