1. **State the problem:** A packet of crayons and an exercise book together cost 2.70. A packet of crayons costs 1.25. We need to find the cost of the exercise book. 2. **Set up the equation:** Let the cost of the exercise book be $x$. The total cost is given by: $$1.25 + x = 2.70$$ 3. **Solve for $x$:** Subtract 1.25 from both sides: $$\cancel{1.25} + x - \cancel{1.25} = 2.70 - 1.25$$ $$x = 1.45$$ 4. **Answer:** The exercise book costs **1.45**.