What is 20 + 10? (nobody answer this)

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?

Leonardo wrote an equation that has an infinite number of solutions. one of the terms in leonardo’s equation is missing, as shown below.

Lines a and b are perpendicular. if the slope of line a is 3, what is the slope of line b?