Please help! How much is 840 miles in 3 & 1/2 hours?

I'm assuming your teacher wants you to calculate the speed

1/2 = 0.5

3 & 1/2 = 3 + 1/2 = 3 + 0.5 = 3.5

If a plane flies 840 miles in 3.5 hours, then,

speed = distance/time

speed = (840 mi)/(3.5 hr)

speed = (840/3.5) mph

speed = 240 mph

240 mph

the law of definite proportions states samples of a compound will always contain the same proportion of elements by mass. in other words, a given compound will always contain the same elements in the same proportions by mass.the law of multiple proportions states when two elements combine with each other to form more than one compound, then the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers.