This is a good one. You have to find out what the words give you.

The sum of the three is 84 tells you

a + b + c = 84

The second number is twice the first tells you

b = 2a

and the third is 4 more than the second tells you

c = 2a + 4

------

a + b + c = 84

b = 2a

c = 2a + 4

Now make the first equation only have the letter a in it.

A + (2a) + (2a +4) = 84

a + 2a + 2a + 4 = 84

5a + 4 = 84

5a + 4 - 4 = 84 - 4

5a = 80

5a/5 = 80/5

a = 16

Use this into the other two.

B = 2a = 2(16) = 32

c = 2a + 4 = 2(16) + 4 = 36

Check your answers

a + b + c = 84

16 + 32 + 36 = 84

84 = 84

The sum of the three is 84 tells you

a + b + c = 84

The second number is twice the first tells you

b = 2a

and the third is 4 more than the second tells you

c = 2a + 4

------

a + b + c = 84

b = 2a

c = 2a + 4

Now make the first equation only have the letter a in it.

A + (2a) + (2a +4) = 84

a + 2a + 2a + 4 = 84

5a + 4 = 84

5a + 4 - 4 = 84 - 4

5a = 80

5a/5 = 80/5

a = 16

Use this into the other two.

B = 2a = 2(16) = 32

c = 2a + 4 = 2(16) + 4 = 36

Check your answers

a + b + c = 84

16 + 32 + 36 = 84

84 = 84