Answer:
y = 16.8
Step-by-step explanation:
Given that y varies directly with x then the equation relating them is
y = kx ← k is the constant of variation
To find k use the condition x = 15 when y = 12, that is
12 = 15k ( divide both sides by 15 )
[tex]\frac{12}{15}[/tex] = k, so
k = 0.8 y = 0.8x ← equation of variation
When x = 21, then
y = 0.8 × 21 = 16.8
