The time it will take Jack and Chris to program the app when they work together is 4.24 hours.
Rate is a ratio that compares two different quantities which have different units.
Jack program an app in 9 hours. Therefore, the rate is as follows;
1 / 9
Chris can program the same app in 8 hours. Therefore, the rate is as follows;
1 / 8
When they do it together.
1 / 9 + 1 / 8 = 8 + 9/ 72 = 17 / 72
1 / t = 17 / 72
cross multiply
72 = 17 t
t = 72 / 17
t = 4.23529411765
t = 4.24
Therefore, the time it takes for both of them is 4.24 hours.
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