a:b is usually read as a ratio. (It can also be read as "odds"; e.g. the odds of winning a sweepstakes prize is listed as 100,000:1 or 100,000 to 1.)

Assuming this is a ratio:

(1) We can read this using order of operations. We treat the ":" symbol as a grouping symbol.

Then (13+4)+20:5 is `(13+4)+20/5=17+4=21`

(2) If by the context of the problem we wanted the ratio of the left side to the right side we have:

[(13+4)+20]:4 or 37:4 which can be written as `37/4,8 3/4,8.75`