To find the compound interest of an investment you have to use this formula, A = P(1 + r/n)^nt, where A is the total amount you have after the investment period, P is the amount you invest or the amount you put in, r is the rate of the of the compound interest in this case 10%, n is the amount of time the interest will be compounded for example, 4 months a year(quarterly) or 6 months a year(semi annually), and t is the amount of time you invest in years.
So in this case you are going to substitute everything in the formula with their given value. So P = $700, r = 10%, n = 21 (because it is the number of months we invest for), and t = 2 years (because 21 months fit perfectly in 2 years, and t must always be in years). The resulting formula will be A = $700(1 + 0.1/21)^(21 x 2), which will give you an answer of $855 rounded to the nearest dollar.
