Yvaries directly with x, and y = 15 when x = 3. what is the value of x when y = 4?

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Y= mx
15= m(3)
m= 5

when y =4
4= 5 (x)
x= 0.8 or 4/5

Y varies directly as x means:
 
y α x 

y = kx   where k is constant of proportionality

y/x = k

when y = 15, x = 3,           y/x = k    

15/3 = k 

5 = k

k = 5.

Now when y = 4,  x = ?

Recall:     y/x = k,     k = 5

4/x = 5

5x = 4

x = 4/5

Therefore x = 4/5

I hope this explains it.

No solution : there is no number of boxes that she bought