Q1: Give two reasons to get good at doing math in your head.
Q2: Give two situations where you might need or want to do math in your head, not with a calculator.
Q3: Explain the quick way to multiply by ten.
Q4: Solve these problems without using a calculator.
(a) 4 x 10
(b) 15 x 10
(c) 400 x 10
(d) 23 x 10
(e) 117 x 10
Q5: Explain the quick way to multiply a single digit times 9.
Q6: Solve the following problems without a calculator.
(a) 4 x 9
(b) 9 x 9
(c) 9 x 8
(d) 5 x 9
(e) 7 x 9
(f) 9 x 2
(g) 3 x 9
(h) 6 x 9
Q7: What's the shortcut to multiply an even number by 5?
Q8: What's the shortcut to multiply an odd number by 5?
Q9: Solve these problems without a calculator.
(a) 5 x 4
(b) 5 x 12
(c) 5 x 19
(d) 5 x 15
(e) 5 x 7
(f) 5 x 20
(g) 5 x 13
(h) 5 x 21
Q10: Don't solve these problems. Instead, check whether each answer could be right or not. Explain your answer.
Example: 4 x 5 = 25
No. An even number x 5 should equal a number that ends in 0.
(a) 11 x 5 = 50
(b) 3 x 5 = 18
Q11: Explain the trick about multiplying by 3.
Q12: How might you get “tricked” by this trick?
Q13: How do you use an easy problem as a shortcut for a hard one? Write an example that shows how.
Q14: Write an example that shows how to multiply by chunks.
Q15: Solve this problem by using one of the tricks you learned in this lesson. Show your work.
49 x 4
Q16: Solve these problems by using one of the tricks. Do them in your head.
(a) 13 x 6
(b) 71 x 7
(c) 19 x 9
Q17: Solve these problems by using one of the tricks you learned in this unit. Show your work.
(a) 11 x 7
(b) 39 x 3
Q18: Solve these problems by using one of the tricks. Do them all in your head.
(a) 9 x 8
(b) 8 x 5
(c) 6 x 5
(e) 3 x 9
(f) 51 x 4
(g) 301 x 10
(h) 7 x 9
(i) 19 x 7
Q19: Don't solve these problems. Instead, circle which answers can't be correct. Explain why.
(a) 8 x 5 = 44
(b) 9 x 6 = 56
(c) 21 x 3 = 61
(e) 9 x 5 = 55
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