L5 · Solve proportional relationship problems

1. 1.

   Solve: 2/x = 312. Find x.

   Answer. 8

   Cross multiply: 2 × 12 = 3x → 24 = 3x → x = 8.

2. 2.

   Solve: 2/x = 936. Find x.

   Answer. 8

   Cross multiply: 2 × 36 = 9x → 72 = 9x → x = 8.

3. 3.

   Solve: 9/x = 954. Find x.

   Answer. 54

   Cross multiply: 9 × 54 = 9x → 486 = 9x → x = 54.

4. 4.

   Solve: 4/x = 36. Find x.

   Answer. 8

   Cross multiply: 4 × 6 = 3x → 24 = 3x → x = 8.

5. 5.

   Solve: 5/x = 618. Find x. Then Mei needs 4 of that amount. What is the total they need?

   Answer. 60

   Cross multiply: 5 × 18 = 6x → 90 = 6x → x = 15. Then multiply by 4: 60.

6. 6.

   Solve: 4/x = 1272. Find x. Then Nora needs 5 of that amount. What is the total they need?

   Answer. 120

   Cross multiply: 4 × 72 = 12x → 288 = 12x → x = 24. Then multiply by 5: 120.

7. 7.

   Solve: 8/x = 1260. Find x. Then Theo needs 2 of that amount. What is the total they need?

   Answer. 80

   Cross multiply: 8 × 60 = 12x → 480 = 12x → x = 40. Then multiply by 2: 80.

8. 8.

   Solve: 5/x = 1248. Find x. Then Luis needs 3 of that amount. What is the total they need?

   Answer. 60

   Cross multiply: 5 × 48 = 12x → 240 = 12x → x = 20. Then multiply by 3: 60.

9. 9.

   Solve: 4/x = 945. Find x. Then Sam needs 3 of that amount. What is the total they need?

   Answer. 60

   Cross multiply: 4 × 45 = 9x → 180 = 9x → x = 20. Then multiply by 3: 60.

10. 10.

    Solve: 6/x = 318. Find x. Then Luis needs 2 of that amount. What is the total they need?

    Answer. 72

    Cross multiply: 6 × 18 = 3x → 108 = 3x → x = 36. Then multiply by 2: 72.

11. 11.

    Think

    Nora tried this problem:
    Solve: 6/x = 1040. Find x.
    Nora wrote -24. That is not correct. What is the right answer?

    Answer. 24

    Nora’s answer -24 is off. Cross multiply: 6 × 40 = 10x → 240 = 10x → x = 24.

12. 12.

    Think

    Sam tried this problem:
    Solve: 8/x = 1040. Find x.
    Sam wrote 64. That is not correct. What is the right answer?

    Answer. 32

    Sam’s answer 64 is off. Cross multiply: 8 × 40 = 10x → 320 = 10x → x = 32.

Original extra practice. Keys are computed from the items, not guessed.