Let Y = number of yellow marbles
R = number of red marbles
"16 in total" means:
Y + R = 16
"number of yellow marbles is 1 more than 2 times the number of red marbles" means:
Y = 1 + 2R
Now, you have a very important choice:
a) you may solve for Y
b) you may solve for R and then remember to solve for Y (asked by the problem)
Let's substitute for R and solve for Y directly.
R = 16 - Y [from first equation]
Y = 1 + 2(16-Y) [from second equation]
Y = 1 + 32 - 2Y
3Y = 33
Y = 11
There are 11 Yellow marbles.
From Y+R = 16, there are 5 Red marbles
Is 11 = 1 + 2(5) ?
11 = 11 ?yes