Arithmetic Grade Four

Lesson Forty-two: Missing Numbers

Study the following problems. 

   __ x 7 = 21		4 x __ = 28		8 x __ = 40

  15 / __ = 5		10 / __ = 2		__ / 7 = 5
In each case we want to put a number in the blank space which will make the number sentence true. We want to complete the number sentence so that we wind up with a true statement.

So let's look at each number sentence from above and fill in the missing number. 


  __ x 7 = 21,  missing number = 3, 	3 x 7 = 21

  4 x __ = 28,  missing number = 7	4 x 7 = 28

  8 x __ = 40,  missing number = 5	8 x 5 = 40

  15 / __ = 5,  missing number = 3	15 / 3 = 5

  10 / __ = 2,  missing number = 5	10 / 5 = 2

  __ / 7 = 5,   missing number = 35	35 / 7 = 5
You can write reverse problems by converting multiplication problems to division problems and switching the numbers around in division problems or converting division problems into multiplication problems. 

	7 x 7 = 49	and	49 / 7 = 7

	48 / 6 = 8	and	48 / 8 = 6

	20 / 4 = 5	and	4 x 5 = 20
This may give you an idea of how you can discover the identity of missing numbers in problems like the ones shown above. 

  If you are given:		Convert it to:		Answer:

    8 x __ = 16			  16 / 8 = __		  2

    35 / __ = 7			  35 / 7 = __		  5

    __ / 4 = 16			  16 / 4 = __		  4

    __ x 3 = 27			  27 / 3 = __		  9
You get to try a few for yourself on the next page.

Continue to next page of lesson