math practice sheet

Any sites you know of would be very helpful, please and thank you!

Okay, so I'm doing this PSAT practice math sheet, and I didn't understand 6 out of 20 of the questions. I got answers that I believed was correct, but weren't. So I was wondering, if someone could tell me why the answers are the answers to the following questions;......... 3x+5y=4z y=4 1.) If x, y and z satisfy the equations above and if 4z=20, what is the value of x? For this problem I got 0, but the answer is 4, how/why? 2.)What is 50 percent of (10+6n)? For this problem, I got 5+3n, but the answer is 5+n. How/why? 3.)What is an equation of the line parallel to the y-axis and three units to the right of the y-axis in the xy-plane? For this problem, I got x=3, but the answer is y=3. How/why? 4.)The clock shows a time of 5:00. If the clock keeps accurate time, what time will it show in exactly 125 hours? For this problem I got 10:00, and the answer is 3:00. Why/how? 5.)If (r-3) to the second power = 0, what is the value of (r+2)(r+4)? This problem was multiple choice, and the choices were, 35, 8, 6, -1, or it cannot be determined from the information given. I got that I could not be determined, but the key said the answer is 6. How/why? 6.) If x < -4 or x > 4, which of the following must be true? I. x to the second power is > 4 II. |x| > 4 III. x to the third power > 4 I wasn't sure how to read the problem, but apparently the answer is I and III are true. How/why? Thank you for helping me, but I really need to know how and why I got those questions wrong. And how to figure them out, so i don't get them wrong in the future.

Evaluate the expression when x = 2, y = 3, z = 4: x + 2 = Question 2: Evaluate the expression when x = 2, y = 3, z = 4: 2y ? 3 = Question 3: Evaluate the expression when x = 2, y = 3, z = 4: xz = Question 4: Evaluate the expression using order of operations: (6 - 4) / (1 + 1) = Question 5: Evaluate the expression using order of operations: 6(9 - 3) + 1 = Question 6: Evaluate the expression using order of operations: 4 + 10/2 - 3 × 2 = Question 7: Evaluate each expression when x = 3, y = 4, z = 6: 4y ? 6 = Question 8: Evaluate each expression when x = 3, y = 4, z = 6: 2(z - x) + y = Question 9: Evaluate each expression when x = 3, y = 4, z = 6: 6 / (z - x) + 4 = Question 10: On a sheet of paper, make a number line. Use it to compare the following numbers, then write the symbol that is appropriate: <, >, or =. 0 __ -4 Question 11: On a sheet of paper, make a number line. Use it to compare the following numbers, then write the symbol that is appropriate: <, >, or =. -1 __ -2 Question 12: On a sheet of paper, make a number line. Use it to compare the following numbers, then write the symbol that is appropriate: <, >, or =. 1 __ -2 Question 13: Evaluate the following expression: 13 + 12 = Question 14: Evaluate the following expression: 22 = Question 15: Evaluate the following expression: 22 - 13 + 12 = Question 16: Evaluate the following expression: (-4)2 = Question 17: Evaluate the following expression when x = 2 and y = 4: 3x2 = Question 18: Evaluate the following expression when x = 2 and y = 4: (3x)2 = Question 19: Evaluate the following expression when x = 2 and y = 4: y2 - (2x) = Question 20: Evaluate the following expression when x = 2 and y = 4: (y - x)3 =

It would be greatly appreciated!! I have two kids that are going to school one is a Kindergarden and the other one is in 4th grade..She needs alot of help in Math...