1. **State the problem:** Melissa wants to find the cost of sending a letter in 1990 using the information about the cost of sending postcards and letters. 2. **Identify the variables:** Let $x$ = cost of sending one postcard. Let $y$ = cost of sending one letter. 3. **List the given information:** - Sending 13 postcards and 7 letters costs 3.70. - Sending 6 postcards and 7 letters costs 2.65. 4. **Write the system of equations:** $$13x + 7y = 3.70$$ $$6x + 7y = 2.65$$ 5. **Use elimination to solve:** Subtract the second equation from the first to eliminate $y$: $$ (13x + 7y) - (6x + 7y) = 3.70 - 2.65 $$ $$ 13x - 6x + 7y - 7y = 1.05 $$ $$ 7x = 1.05 $$ 6. **Solve for $x$:** $$ x = \frac{1.05}{7} = 0.15 $$ 7. **Substitute $x=0.15$ into one of the original equations to find $y$:** Using $$6x + 7y = 2.65$$: $$6(0.15) + 7y = 2.65$$ $$0.90 + 7y = 2.65$$ $$7y = 2.65 - 0.90 = 1.75$$ $$y = \frac{1.75}{7} = 0.25$$ **Final answer:** The cost to send a letter in 1990 was $0.25$.