15. When the same two digits are next to each other in a number, the value of the number to the left is ten times the value of the one to the right. This is because we count in base ten and each place value represents the digit multiplied by 10^x.
16. 5,771 The one in the hundred place has a value of 7*100 which is 700. The one in the tens place has a value of 7*10 which is 70. The answer is B.
The ratio of boys to girls is 4:5. So let 4x be the number of boys and 5x be the number of girls. Then 4x+5x = 9x is the total number of students. Then the fraction of boys in the class is 4x/9x = 4/9.
