What proportion of this population true-breeding double | Homework Helpers

Background:

A geneticist discovers two genes that are involved in the cell cycle of C. elegans, a commonly used model organism. She studied the effects of these mutations in separate worm lines: aab+b+ and a+a+bb. to gain a better understanding of the cell cycle, she needs to create a double mutant line that lacks functional alleles for either gene (aabb). The genome project has discovered tah these two genes are on the same chromosome, 8 map units apart.

Problem:

She crosses the two lines together (aab+b+ x a+a+bb) to create a heterozygote (a+ab+b). Two heterozygotes are crossed together to create an F2 population. What proportion of this population will be true-breeding double mutant?