If you need an estimate for #5, then you can round each value to the nearest whole number to have...
- 21.6 turn into 21
- 20.9 turn into 21
- 19.4 turn into 19
- 18.5 turn into 19
Add up those values: 21+21+19+19 = 80
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Another estimate you could do is to round everything to 20, since we're close to this value. Having four copies of 20 added leads to 20+20+20+20 = 80
So they rode a total estimated amount of 80 miles
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The actual total is: 21.6+20.9+19.4+18.5 = 80.4 miles, which isn't too far off from 80.