for this you would use a punnet square (ill try my best to show it while typing)
heterozygous means different, so you have parents with alleles shown as Dr (D=dominate, r= recessive) since they cant have 2 dominate alleles.
D r
D Dr Dr
r Dr rr
to get the recessive trait you must have an allele shown recessive homozygous
since there are only 1 pair shown as rr out of 4 possibilities you'd have 1/4 which is 25%
25%