I'm using system of equations and I have to use either substitution or elimination. My two equations are 2x - 6y = 13
8x - 24y = 8

In a class, there are 12 girls. if the ratio of boys to girls is 5: 4 how many boys are there in the class

At the beginning of 1974/75,2 years a man invested #2000 in a bank at 12.5% compound interest, if it degree #650 at the end of each two years to pay his son's school fees, after payment of interest to pay his son's school fees, how much did he have left at the beginning of the 1978/79 school year?

Math it's appreciated. (picture provided)