So I'm guessing that the unknowns here would be the number of postcards and letters that the couple sends to each other. So let's represent a letter with L and a postcard with the letter P. To get the equation of henry and isobel, you just need to plot the variables and the numbers. So the equation of henry is L + P = 150 because he only sent one letter and one postcard that made a total of 150. Isobel's equation is 5P + L =390 because isobel sent five postcards hence 5P, and one letter to make a total cost of 390.