its D 22 bottles
basically i just assumed small bottles were 1 dollar and large were 3 dollars then found the budget the first dude that bought 16 bottles $(1 x 10 + 3 x 6)= $28. and used that same budget to determine the girls amount of small bottles using her limit of 24 bottles as a constraint.
ok for the way with equations. this problem can be expressed as 2 equations with 2 unknown varibles. then we can use our algebra to solve
first equation (still using the price of bottles as 1$ for small 3$ for large)
since we know bill or dave or watever spent 28 dollars on his bottles we can express that as
here i am using x as the small bottles and y as the big bottles.
now for the second equation since we know that lisa has to buy 24 bottles (a mix of small (x) and large (y) bottles)
now we solve using these 2 equations.
if we multiply the bottom equation by (-1) and then add the 2 equations we are able to solve for y. once you obtain this value for y you can plug that value into either of the 2 equations to obtain x (which is the amount of small bottles lisa bought, and your answer)