Resources

Use the cards from the mental routines

The teacher was trying to sort the cards so that each set had 40% of the feet belonging to crabs, 20% to insects and dogs. 15% of the feet should belong to snails and the remaining 25 % to a mixture spiders and boys.
She wanted more spiders than boys and
more dogs than insects.
How can she, make this work?
How many cards will she need altogether?

The Activity

As the students use the cards or other means of working on this problem it is hoped that they will soon see that there must be more than 100 feet or there will be an odd number of snail's feet (15% of 100 is 15) and that could not work out. Encourage the students to pose somewhat ifs that might help them move forward, for instance what if there were 50 legs-no good because then there would be half a snail. You may need to remind the students that the question asks for the number of cards. This means that once the students have identified that 200 legs would work for the percentages given they still need to work out how many of each type of creature there are and then how many cards.

Reflection

Use the reflection time to look at the range of strategies used. Some students may have used 1% 0r 10% as their methods for working out how many of each card there are whereas others may have converted 25% or 20% to fractions to work out the number of feet each time. It is also possible that some students will have worked out the percentages for 100 feet and then doubled the results. Discuss the similarities and effectiveness of each strategy and ask the students to say which they would use in future percentage problems and to explain why.