From a bar to x

Algebra is the bar model written in letters. Call the unknown bar x, write down what the drawing shows, and you have an equation.

In the Ana and Ben problem, Ben's bar is x. Ana's bar is x and an extra 15. Both bars together are 85. So x + x + 15 = 85, which is 2x + 15 = 85.

Solving the equation is the same as the drawing: take the 15 away from both sides, then share what is left by two. A child who can draw it can usually follow the letters.