Finally, you got one wrong.  The correct answer is 174.  The easiest way to solve this problem is to first find 10% by moving the decimal point over one space, so 870 becomes 87.  Twenty percent is twice that much so double 87, and the answer is 174.

I recommend starting with Volume 2 of the Learn Math Fast System.  That book will only take about a month to complete, and you'll learn how to quickly solve percentages, decimal numbers, fractions and negative numbers.  Use the coupon code PT2026 get 10% off your order.