Atennis instructor chargers $40 for each one hour lesson and $25 for each half hour lesson if the instructors i come from 35 lessons was $1115 how many one hour lessons did she give?

X = 1 hr lessons and y = 1/2 hr lessons

x + y = 35y = 35 - x
40x + 25y = 1115

40x + 25(35 - x) = 1115
40x + 875 - 25x = 1115
40x - 25x = 1115 - 875
15x = 240
x = 240/15
x = 16 <=== there are 16 one-hr lessons
and 19 half-hr lessons

Aand b are correctly balanced reactions. they have the same number of atoms in both sides of the reaction