Question

Y varies directly as x, y = 7 when x = 21. Determine x when y = 5.

Answer 1

y varies directly as x, y = 7 when x = 21. Determine x when y = 5.

Step 1

Let

y varies directly as x, it is y depends on x, in math terms

f(x)=y

y = 7 when x = 21

f(21)=7

Determine x when y = 5. f(?)=5

Step 2

there is a proportion, this must be equal, make a rule of three to find the value

so

x y

`\begin{gathered} 21\leftrightarrow7 \\ x\text{ }\leftrightarrow5 \\ \text{the relation is} \\ (21)/(7)=(x)/(5) \\ \text{solve for x} \\ x=(21\cdot5)/(7) \\ x=(105)/(7) \\ x=15 \end{gathered}`

so , when y=5, x=15