Okay i will be using a for adults and c for children. so the equation would be 2a + 3c = 42 since we know the kids dinner costs $4 we can replace c with 4 so the new equation is 2a + 3(4) = 42
3 x 4 = 12 so now you can write it as 2a + 12 = 42 now subtract 12: 2a + (12-12) = 42 - 12
now 2a = 30 now you just need to divide 30 by 2 which is 15 so the final answer is the adult dinners cost $15 each
you can check the answer by doing: 2(15) + 3(4) = 42 30 + 12 = 42 42 = 42
