An estate has 6 houses and each house has x lighting fittings which need 1 lamp each, and y fittings which need 3 lamps each. Write a formula to find z, the total number of lamps needed on the estate.
z = 6(x * 1 + 3 * y)
z = 6(x + 3y)
z = 6(x * 1 + 3 * y)
z = 6(x + 3y)