Olivia has $180 to buy presents for her 3 sisters and 2 nephews. she wants to spend $15 more on each sister than she spends on each nephew. how much does she spend on each sister?

So
if x is the number she spent on each sister and y is the number she spent on each nephew then
180=3x+2y

she wants to spend 15 dollars more on sisters than nephews
so
x=15+y
solve

we have
180=3x+2y and
x=15+y
subsitute 15+y for x

180=3(15+y)+2y
180=45+3y+2y
180=45+5y
minus 45 both sides
135=5y
divide both sides by 5
27=y

solve for x
x=15+y
x=15+27
x=42

she spent $42 on each sister