Njuice8

be more careful when you interptert formulas in math...The problem you mentioned above is a typical problem of summing an arithmetic sequence with first term a=20 and ratio t=4 the formula that gives the sum of n terms is (n/2)[2a+(n-1)t] and not the one you wrote ..equate this to 800 and solve a quadratic for n to find n =16 .

Good Luck

MINOAS