What is the missing number 21 plus what equals 71?
The number 21 has two tens and one one. The number 71 has seven tens and one one. We need to work out what was added to 21 to make 71. The ones digit in these two numbers has stayed the same, so we didn’t have to add anything to the ones. What do we need to add to two tens to make seven tens? We know that two plus five makes seven. So, two tens plus five tens makes seven tens. So, the missing number is 50. 21 plus 50 equals 71.