We can find the value of an expression when given the values of \(x\), \(y\), etc.

Before we start, we will need to remind ourselves of BODMAS: the order in which we carry out calculations.

- Question
Find the value of \(3x + 5y\) when \(x = 2\) and \(y = -1\).

To find the value of an expression, we need to substitute the given values for \(x\) and \(y\) into \(3x + 5y\).

\[3x + 5y\]

\[ = 3 \times 2 + 5 \times ( - 1)\]

Remember, BODMAS tells us to multiply before we add.

\[= 6 + ( - 5)\]

\[= 1\]

- Question
Evaluate \(5c - b + 2a\) when \(a = 3\), \(b = 1\) and \(c = 4\).

\[5c - b + 2a\]

\[= 5 \times 4 - 1 + 2 \times 3\]

\[= 20 - 1 + 6\]

\[= 25\]

- Question
An approximate formula which converts celsius temperatures to farenheit is \(F = 2C + 30\).

Calculate \(F\) when \(C\) is 15.

\[F = 2C + 30\]

\[F = 2 \times 15 + 30\]

\[F = 30 + 30\]

\[F = 60\]

Therefore 15°C is approximately equal to 60°F.