In order to keep the songbirds in the back yard happy, Sara puts out30g of seeds at the end of each week. During the week, the birds find and eat3/4 of the available seeds. We set up a DTDS S_t+1=f(S_t) for the amount of seeds in the backyard, where t is measured in weeks and seeds are counted just after a new suply is provided.

I found the updating function, which is 0.25*S+30 and the fixed point of the DTDS is 40. My question is:

Find the general solution formula for the DTDS.