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Washing Elephants

Age 11 to 14 Short
Challenge Level

Answer: 22$\frac29$ minutes


Scaling up to more elephants
Mary: 50 minutes   1 elephant          200 minutes   4 elephants

Nigel: 40 minutes   1 elephant          200 minutes   5 elephants

Together:  200 minutes    9 elephants
             200$\div$9 minutes  1 elephant
             200$\div$9 = 22$\frac29$


Seeing how much of the elephant is clean after 10 minutes (or 5, or 20, etc)
Mary:  50 minutes   1 elephant            10 minutes   $\frac15$ elephant

Nigel:  40 minutes   1 elephant            10 minutes   $\frac14$ elephant

Together:                                            10 minutes   $\frac15+\frac14=\frac9{20}$ elephant
                                                       10$\div$9 minutes    $\frac1{20}$ elephant
                                                      200$\div$9 minutes    1 elephant
                                                      200$\div$9 = 22$\frac29$


Finding the amount of elephant washed by each sibling
Mary:  50 minutes   1 elephant            10 minutes   $\frac15$ elephant

Nigel:  40 minutes   1 elephant            10 minutes   $\frac14$ elephant

amount of elephant that Nigel washes $:$ amount of elephant that Mary washes
                                                    $\frac14:\frac15$
                                                     $5:4$

$\therefore$ Nigel will wash $\frac49$ of the elephant (and Mary will wash $\frac59$ of the elephant).
It will take $\frac49\times$40 = 22$\frac29$ minutes

 

You can find more short problems, arranged by curriculum topic, in our short problems collection.