Do it one piece at a time. Seven tens is 70. Nine ones is 9.Now add 70 + 9 and you get 79.
90 + 7 is not and has never been equal to 70 + 9
70 tens = 70 x 10 = 700
700 ÷ 10 = 70
700 hundreds is 70,000. 70,000 contains 7,000 tens.
70
702 tens.
70
9 tens are = to 90.
7
7 tens, or 70.
70 of them.