I have decided to do this strategy. This was the only one I didn't understand. I finally figured it out.
I have learnt that if you add the same amount to the equation. The answer will still come out the same .
Paris and her brother spent $183 in their sisters 21st birthday present. Her brother paid $68 towards it. How much did Paris pay ?.
183 - 68 = 115
If you add 2 to both numbers it will be 185 - 70 and it will still equal the same amount .