Let A = adults, C = children

You generate 2 equations.

One for the total number of people A + C = 440

And one to factor the costs 2.50A + 1.25C = 630.00

Now that you have 2 systems of equations you can substitute and solve.

A = 440 - C

substitute that expression for A into the 2nd equation.

2.50(440 - C) + 1.25C = 630

Expand it out and combine like terms.

1100.0 - 1.25C = 630

subtract 1100 from both sides

-1.25C = -470

C = 376

Using the fact that A = 440 - C

A = 64.

64 Adults, 376 Children