Math Is Fun Forum

elayne

Guest

math

I am trying to help my daughter understandthis word problem but I dont know how to figure out how to convert a mixed number into time/hrs.

the question is if it takes me 4/5 of an hour to bake acake and 5/8 of an hour to clen my room what is the total time it will take me to do both

Please help

Jai Ganesh

Administrator

Registered: 2005-06-28

Posts: 46,328

Re: math

The solution is
4/5 hr+5/8 hr = (32+25)/40=57/40 hours  = 1 17/40 hours that is 1 hour and 1530 seconds.
However, if baking does not involve any work, it would take only 4/5 of an hour to do both, that is 48 minutes.

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NullRoot

Member

Registered: 2007-11-19

Posts: 162

Re: math

Think of the 4/5 and 5/8 as percentages. 4/5 and 5/8 convert to nice decimals; 0.8 (or 80%) and 0.625 (or 62.5%) respectively.

To find out what 80% of an hour is, you need to put it into other terms. An hour is 60 minutes so you need to find out what 80% of 60 is. You do that by calculating 0.8 * 60 minutes. It comes out to 48 minutes.

Do the same thing for the other one. 0.625 * 60 turns out to be 37.5 minutes.

You might know off the top of your head that 0.5 minutes is 30 seconds, but you can calculate it in the same way. 1 minute = 60 seconds. 0.5 * 60 seconds = 30 seconds.

So to bake it will take you 48 minutes. To clean your room will take you 37 minutes and 30 seconds.
As Ganesh points out, if you can leave the cake to bake and clean your room at the same time, then it will take 48 minutes. (and you get 10 minutes, 30 seconds to rest )

However, if you have to do them one after the other, then it's 48 minutes + 37 minutes + 30 seconds, which is 85 minutes, 30 seconds.

You can then subtract 60 minutes and add on an hour if you wish so that your total time is 1 hour, 35 minutes, 30 seconds.

Best of luck with your math.

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John E. Franklin

Member

Registered: 2005-08-29

Posts: 3,588

Re: math

Don't forget while the cake is baking, you can start to clean the room and then wash your hands and check on the cake, thus saving 5 or 10 minutes.
And 4 ÷ 5 is 80% and 5 ÷ 8 is 62.5%, they total 142.5% hours.

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