[tex]10 \times (a + b) = (c \times 5) + (d \times 7) \\ 10 \times a + 10 \times b = c \times 5 + d \times 7 \\ 2 \times 10 \times a + b = c \times 5 + d \times 7 \\ 20 \times a + b = 5 \times 7 + c \times d \\ 20 \times a + b = 35 + c \times d \\ a + b = 35 + c \times d \div 20 \\ [/tex]
