Write the missing number. Then,give the property used in each equation
1. 50+90=___+50___
2. 7×(25+5)=(7×25)+(7× ___)____
3. (10+15)+20=10+(__+___) _____
4. (8×___)×9=__×(5×9)____
5. 50+20+15=15+__+50:____.