Given that a = 15 cm and b = 8 cm, work out x

Donald has x twenty-dollar bills and 11 ten-dollar bill. how much money does donald have? write your answer as an expression.

Atrain leaves little? rock, arkansas, and travels north at 90 kilometers per hour. another train leaves at the same time and travels south at 80 kilometers per hour. how long will it take before they are 680 kilometers? apart?

Gabrielle's age is two times mikhail's age. the sum of their ages is 84. what is mikhail's age?