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Age 5 to 7 Challenge Level:

91 is on the back of 100

53 is on the back of 58

28 is on the back of 23

12 is on the back of 19

 

Here are some patterns you may have noticed:

  • The tens in each pair don't change ie 58−53, both 5 tens.
  • The units in each pair add up to 11.
  • 100 and 91 look different. 100 has a zero in the tens column, and 90 has a nine. In the units, 1+0 doesn't = 11. But 100=9 tens & 10 units. 91=9 tens & 1 unit. Now the tens stay the same, and the units add up to 11.