I am sorry I didn't finish my thought up above...I totally got that the first line he did in his head...and the last line in my example would be the conclusion that anyone can plainly see from 70 + 10 = 80. He compared one side of the equation to the other and made it equal. Perfectly fine answer to me mathematically.

...reading is pleasure, not just something teachers make you do in school.~B. Cleary