## Tuesday, 24 February 2015

### Substitution: Can you tell What's Wrong? (2)

1. You cannot cross out during subtraction and you can only do this during multiplication and division.

1. OR cross division to simplify the problem

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3. When working your solution, your first step should be, where possible, to convert the letters into numbers.Therefore the first step should be a/b - b/c = 3/2 -2/-1.

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2. The mistake is in line 1.

Group 1

4. You cannot do cross division in subtraction. You should instead change the denominators to be the same, then subtract.

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5. Crossing is only for multiplication.So line 1 is wrong

6. Crossing is only for multiplication.So line 1 is wrong

1. My suggestion is to subsitite the letter for the number so it is easier

7. you cant cross out in subtraction and can only be done in mutiplication. b and c should be multiply to the same number then subtract

8. you cant use the cross out method in subtracting fractions you should use that only in multiplication

1. you should substitute all the letters to numbers first for more clarification

9. You only need to cross out when you do multiplication or division, not when you are doing subtraction and addition. Then from line 1 to line 2, "c" did not change to a negative number.

10. You cannot cross out during subtraction of decimals. You should change the denominators to be the same and then subtract. In line 2.

11. In line 2, the person who solved the question slashed out the "bs" and replaced them with 1s, but this is wrong because he was supposed to subtract the fractions, and you cannot slash it out unless you are supposed to multiply the fractions. In line 3, when he converted the letter c to a number, he forgot the - sign, so it was not negative anymore.

12. You cannot cross out during subtraction of decimals. You should change the denominators to be the same and then subtract. In line 2.

13. The person should have converted the letters into numbers first instead. They also should only cross out numbers as they crossed out the letters instead. Also, since it is subtraction, there should be no crossing out of numbers.

14. He should have substituted the variables to numbers in line 1.

15. In line 1, they should not have done cross division (cancelling both b's when they were across) as it was not multiplication or division.

16. In line 1, he should have substituted the a out of b to 3 out of 2 first and the b out of c to 2 out of -1.

17. In line 1, he should have substituted the a out of b to 3 out of 2 first and the b out of c to 2 out of -1.

18. he should substitute the letters by numbers before subtracting the equation first instead of crossing out the letter b

19. In line 1, the b cannot be cancelled in that way.

20. They could not cross out the two variables in the question. (cross multiplication) as they are not numbers, but variables

1. The variables can be any number, and they should substitute the variables to numbers first

21. the dude should convert the alphabet to number first before doing the fraction working

22. In line 1, he should substitute the alphabets into numbers. and you should not cross out the numbers during subtraction (line 1). you should make the denominators the same.

23. Crossing out numbers in subtraction is not allowed. only allowed in multiplication

24. Crossing out numbers in subtraction is not allowed. only allowed in multiplication