In a school there are w girls and x boys. y more girls and z more boys joined the school. How many girls are there in the school now?
There are w girls and x boys in the school.

y more girls joined the school.

The number of girls in the school now is w + y.

z more boys joined the school.

The number of boys in the school now is x + z.

The total number of students in the school now is w + y + x + z.

We know that w + y + x + z = w + x + y + z.

Therefore, w + y = w + x + y + z - (x + z) = w + x + y + z - x - z = w + y + z.

The number of girls in the school now is w + y = w + y + z - z = w + y + z - (x + z) + (x + z) - z = w + y + z - z + x + z = w + y + 2x + z.