It takes Billy fifteen minutes to complete 1/8 of a recipe. At this rate go long will it take for him to complete the recipe

well, a WHOLE is 1, in this case is 8/8, that'd be a whole.

so Billy does 1/8 in 15 minutes, well, there are 8/8, so do the whole recipe that'd be 8 * 15 minutes, or 120 minutes, or 2 hours.

or you can see it this way, 1/8 is 15 minutes, since there are eight 1/8's, then it'd take 15+15+15+15+15+15+15+15 minutes.