So,
Let x represent Joe's weight and y represent Jeff's weight.
"Joe weighs 20 lbs. less than twice Jeff's weight."
x = 2y - 20
"If Jeff would gain 10 lbs., then together they would weigh 250 lbs."
(y + 10) + x = 250
We now have our two open sentences.
x = 2y - 20
(y + 10) + x = 250
Get rid of parentheses.
x = 2y - 20
x + y + 10 = 250
We will use Elimination by Substitution.
2y - 20 + y + 10 = 250
Collect Like Terms.
3y - 10 = 250
Add 10 to both sides.
3y = 260
Divide both sides by 3.

Substitute again.

Multiply.

Subtract.

Check.
"Joe weighs 20 lbs. less than twice Jeff's weight."
Jeff's weight times two is 173 and one-third.
20 lbs. less than that is 153 and one-third lbs. Check.
"If Jeff would gain 10 lbs., then together they would weigh 250 lbs."
86 and two-thirds + 10 = 96 and two-thirds.
96 and two-thirds + 153 and one-third equals 250 lbs. Check.

First we need to multiply to make the fractions have equal denominators so we can add/subtract.
math: 1/3 x 4/4=4/12
science: 1/4 x 3/3=3/12
Now we can find the time she spent on history (I assume this is what you are looking for)
Total time-math time<span>-science time=history time</span>
12/12-4/12-3/12
=5/12
Amy spent 5/12 of her time working on history.


side note: multiplying by the LCD of both sides is just to get rid of the denominators
Answer: a)
b) 
<u>Step-by-step explanation:</u>
a) In order to get an even number, you have the 3 different scenarios:
1) Even, Even, Even, Even 
2) Even, Even, Odd, Odd
3) Odd, Odd, Odd, Odd 
<em>Order doesn't matter</em>
Add them up to get your answer: 
b) If one die is a 2 and another is a 3 and the other two dice can be any number, then you have 1 possibility for a 2, 1 possibility for a 3, and 6 possibilities for each of the other two dice.
