Explanation

Use two separate lines in a W = RT chart, one for Wendy and one for Miguel, to calculate their respective rates. Building 1 birdhouse equals doing 1 unit of work:



Thus, Wendy’s rate is $\frac{1}{15}$ birdhouses per hour, and Miguel’s rate is $\frac{1}{10}$birdhouses per hour. Since Wendy and Miguel are working together, add their rates:



Now solve for t by first combining the fractions:

$1=(\frac{1}{15}+\frac{1}{10})t$

$1=(\frac{5}{30})t$

$t=6$