Abe, bella, and carlos have all earned some college credits.  bella has earned 10 less credits than abe, and carlos has earned two less than double the number of abe’s credits.  altogether, they have earned a total of 136 credits.  how many credits have each of them  earned?    

Set Abe as x
set Bella as y
set Carlos as z

Bella has earned 10 less credits than Abe
y=x-10

Carlos has earned two less than double the number of Abe’s credits
z=2x-2

Altogether, they have earned a total of 136 credits
x+y+z=136

replace y with x-10
replace z with 2x-2
x+(x-10)+(2x-2)=136
solve for x
4x-12=136
4x=148
x=37

We now know Abe earned 37 credits
To find how much Bella earned plug in 37 for y=x-10
y=27
To find how much Carlos earned plug in 37 for z=2x-2 
z=72

Therefore,
Abe earned 37 credits
Bela earned 27 credits
Carlos earned 72 credits

Element 17 is chlorine it is a group 7 element(halogen) and belongs to period 3 of the periodic table