Make doubles. add. 5+6 __+_+_ so,5+6=11

If you want 2 numbers, you have: 1+10, 2+9, 3+8, and so on. If you need 3 numbers you have 1+1+9, 1+2+8, 2+2+7, and so on.

The doubles plus one are the facts in which one addend is larger than the other by one. Examples 3+4=7, 6+7=13, 5+6=11, 3+2=5. Learning your doubles plus one should involve knowing the doubles and mentally adding the additional one. ubles are adding the same number together, as with 5+5=10.
With doubles +1, the numbers must be in beside each other in counting order going up or down (ex: 4, 5, 6  and  9, 8, 7). I teach my first graders to say the SMALLEST number, double it, then add one. So, 9+8 would be 8+8=16, +1=17. You must know your doubles facts before this strategy is useful.

The answer is 16.64 feetyour welcome