How many significant figures are there in the answer for the following problem? 56.4 + 0.8822 + 21 =?

Atrain travels 74 kilometers in 3 hours, and then 81 kilometers in 5 hours. what is its average speed?

Given sno2 + 2h2 - sn + 2h20 tin oxide reacts with hydrogen to produce tin and water. how many moles of sno2 are needed to produce 500.0 grams of sn?

How many grams of oxygen gas are contained in a 15 l sample at 1.02 atm and 28°c? show your work.