With the third question, your second equation was a little off.
Here's the answer from the website I got it from:
In three more years, Miguel's grandfather will be six times as old as Miguel was last year. When Miguel's present age is added to his grandfather's present age, the total is 68. How old is each one now? Copyright © Elizabeth Stapel 2000-2011 All Rights Reserved
This exercise refers not only to their present ages, but also to both their ages last year and their ages in three years, so labelling will be very important. I will label Miguel's present age as "m" and his grandfather's present age as "g". Then m + g = 68. Miguel's age "last year" was m – 1. His grandfather's age "in three more years" will be g + 3. The grandfather's "age three years from now" is six times Miguel's "age last year" or, in math:
g + 3 = 6(m – 1)
This gives me two equations with two variables:
m + g = 68
g + 3 = 6(m – 1)
Solving the first equation, I get m = 68 – g. (Note: It's okay to solve for "g = 68 – m", too. The problem will work out a bit differently in the middle, but the answer will be the same at the end.) I'll plug "68 – g" into the second equation in place of "m":
g + 3 = 6m – 6
g + 3 = 6(68 – g) – 6
g + 3 = 408 – 6g – 6
g + 3 = 402 – 6g
g + 6g = 402 – 3
7g = 399
g = 57
Since "g" stands for the grandfather's current age, then the grandfather is 57 years old. Since m + g = 68, then m = 11, and Miguel is presently eleven years old.
There's a few more decent questions on there, the website is:
http://www.purplemath.com/modules/ageprobs.htm
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