Explanation:
we only need to calculate directly the work hours needed for 10 child bikes and 12 adult bikes.
as a child bikes needs 4 hours to build and 4 hours to test, for 10 child bikes that means 10×4 = 40 hours to build and 40 hours to test
an adult bike needs 6 hours to build and 4 his to test.
so, for 12 bikes that are 12×6 = 72 hours to build and 12×4 = 48 hours to test.
together that means we need
40 + 72 = 112 hours to build
40 + 48 = 88 hours to test
the limits of the company are 120 build hours and 100 test hours per week.
as 112 < 120 and 88 < 100, yes, the company can build 10 child bikes and 12 adult bikes in one week.
in fact, with that they still have 8 work hours (120 - 112) and 12 test hours (100 - 88) left and could therefore build either 2 additional child bikes (8 build hours, 8 test hours) or one additional adult bike (6 build hours, 4 test hours).