Question

A bag contains white marbles and blue marbles, 79 in total. the number of white marbles is 6 less than 4 times the number of blue marbles. how many white marbles are there?

Answer 1

Let x be the number of white marbles and y be the number of blue marbles. 
So, $x+y=79$
Also, $x = 4y-6$ since "the number of white marbles is 6 less than 4 times the number of blue marbles"
We will solve the above system o two equation:
Using the value of x from the second equation in the first equation we get:
$(4y-6)+y=79\\5y=79+6\\5y=85\text{ then }y=\frac{85}{5}=17\\x=79-17=62$
Conclusion: there is 62 white marbles and 17 blue marbles.