Answer with explanation:
There are following double facts
When a Number is added to itself the result is twice of that number.
1+1=2 ×1=2
2+2=2×2=4
3 +3=2×3=6
4+4=2×4=8
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Number 15 can be written in two ways
15=14+1-----(1)
=16-1------(2)
So, the double fact that we can use to solve , 8+7=15 are
1.→ 8+7
=14+1
=7+7+1
=(7+7)+1
2. →8+7
=15-1
=(8+8)-1