She charges $15 per lawn, so for mowing x lawns, she earns 15x.
She has a fixed expense of $30 in total, not per lawn.
15x - 30 = 40
15x = 70
x = 4 2/3
If she mows 4 2/3 lawns, she will make a profit of $40, but since she must mow full lawns, not a fraction if it, she must mow 5 lawns.