If x + 9 = 21, what is the value of x?

Subtract 9 from both sides: x = 21 − 9 = 12.

A store sells 6 apples for $1.50. How much do 10 apples cost?

Unit price = $1.50 ÷ 6 = $0.25 each; 10 × $0.25 = $2.50.

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A rectangle has a length of 12 cm and a width of 7 cm. What is its area?