Programm for the TI 89 Titanium

Hello guys,

i hope u can help me with the following problem:

I have this formal task format:

310° = (3,2)°

[317]° = 

320° = (3,6)°

The square bracket number is variable. 

The curved bracket number is given. 

The numbers 310, 320 are easly the boundaries of 317 ° and are every time equal (interval of 10): 

Examples:   214 = 210, 220   ;   184 = 180, 190   ; 43 = 40, 50

Every time the numbers of the boundaries are given and have to fill out!

Now the calculation:

The programm have to calculate the number of 317° with the consideration of the numbers from the boundaries. 

I'm nut sure but is this a sort of interpolation? 

I hope u can understand my problem, because my english is not so good 

May, u can help me with my problem to write a programm, wich can calculate this, it would be great!