A bag contains green marbles and blue marbles, 40 in total. The number of green marbles is 7 more than 2 times the number of blue marbles. How many green marbles are there?

Answer: There are 29 green marbles.

Step-by-step explanation:

x: green marbles

y: blue marbles

So we have an equation:

x + y = 40

while,

2y = x - 7

so set this equation equal to x, we have

x = 2y + 7

replace x into 2y + 7 for the first equation

and this is how it looks:

(2y + 7) + y = 40

2y + y = 40 - 7

3y = 33

y = 11

x = 40 - y

x = 40 - 11

x = 29