In peas, the round allele (r) is dominant to the wrinkled allele (r), and tall (t) is dominant to short (t). a pure-breeding round, tall strain is crossed to a wrinkled, short strain, and the f1 plants are crossed back to the wrinkled, short strain. what proportion of the offspring will be wrinkled and tall

The round allele is dominant (R)
The wrinkled allele is recessive (r)
The tall allele is dominant (T)
The short allele is recessive (t)

The genotype of the f1 generation would be RrTt.So if you cross that back to the wrinkled short then the Punnet of that cross would look like this:

RrTt x rrtt

      RT           Rt           rT          rt
rt   RrTt       Rrtt          rrTt        rrtt
rt   RrTt       Rrtt          rrTt        rrtt
rt   RrTt       Rrtt          rrTt        rrtt
rt   RrTt       Rrtt          rrTt        rrtt

The genotype for a wrinkled tall should be rrTt at least in this case because neither of the parents are pure-breed. 

The proportion would then be then 4/16 or 1/4. There is a 25% chance of  yielding a wrinkled tall.