Answer:
replace the ys with -3 then solve it
Answer: 10
<u>Step-by-step explanation:</u>
varies directly means: 
Step 1: solve for k ⇒ 
Step 2: plug in the given value and k to solve for the missing value: 
42(5) = 21(x)

2(5) = x
10 = x
Answer:
I have no idea
Step-by-step explanation:
I have no idea
Let: x be Amy's marbles
3x be Robin's marbles
15+3x be Sam's marbles
x+3x+15+3x=99
7x+15=99
7x=99-15 (transfer 15 to the right side of the equation, then minus)
7x/7=84/7 (divide both sides of the equation by 7)
x=12
Now substitute:
x = 12 (Amy's)
3x = 3*12 = 36 (Robin's)
15+3x = 15+3*12 = 51 (Sam's)
12+36+51 = 99
Therefore, Amy has 12 marbles, Robin has 36 marbles and Sam has 51 marbles.