Start by putting two coins on each side of the scale. If one side is higher, then the light coin must be in that pair (use a second weighing to determine which of that pair is the lightest.)
If the initial weighing shows the two pairs to be even, discard those four and go to the three others. Put two of the coins on the scale - one on each side. If one side stays higher, that coin is your light one. If however they are equal, the final unweighed coin must be the lighter coin.
Answer:
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The answer is 4.2 because you have to replace r with -14.1 and s with 18.3. After that you subtract them and you get 4.2
4x³=756
x³=189
x=3 ³√7 should be acceptable
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If there are 108 students, and the vans can hold up to 5 students, then you would need 22 vans.
If you were to divide 108 by 5, then you would get 21.3 . That 3 is the remaining children, therefore, to fit them in the trip, you would need another van, hence, 22 vans. The van that is not full would have 3 children in it.