A bag contains white marbles and red marbles, 20 in total. The number of white marbles is 5 more than 2 times the number of red marbles. How many white marbles are there?

Let the number of red marbles be : x

number of white marbles : (2x + 5)

total number of marbles : 2x + 5 + x = 3x + 5

3x + 5 = 20

3x = 20-5 = 15

x = 15/3 = 5

since x = 5

and

white marbles : (2x + 5)

no. of white marbles : 2 (5) + 5 = 15

answer : 15 white marbles