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Fofino [41]
3 years ago
7

A boat that can travel 18 mph in still water can travel 21 miles downstream in the same amount of time that it can travel 15 mil

es upstream. Find the speed (in mph) of the current in the river.
Mathematics
1 answer:
svlad2 [7]3 years ago
3 0

Hey there! I'm happy to help!

We see that if the river isn't moving at all the boat can move at 18 mph (most likely because it has an engine propelling it.)

We want to set up a proportion where our 21 miles downstream time is equal to our 15 miles upstream time so we can find the speed. A proportion is basically showing that two ratios are equal. Since our downstream distance and upstream distance can be done in the same amount of time, we will write it as a proportion.

We want to find the speed of the river. We will use r to represent the speed of the river. When going downstream, the boat will go faster, so it will have a higher mph. So, our speed going down is 18+r. When you are going upstream, it's the opposite, so it will be 18-r.

\frac{distance}{speed} =\frac{21}{18+r} = \frac{15}{18-r}

So, how do we figure out what r is now? Well, one nice thing to know about proportions is that the product of the items diagonal from each other equals the product of the other items. Basically, that means that 15(18+r) is equal to 21(18-r). This is a very nice trick to solve proportions quickly. We see that we have made an equation and now we can solve it!

15(18+r)=21(18-r)

We use the distributive property to undo the parentheses.

270+15r=378-21r

We subtract 270 from both sides.

15r=108-21

We add 21 to both sides.

36r=108

We divide both sides by 36.

r=3

Therefore, the speed of the river is 3 mph.

You also could have noticed that 18mph to 21 mph is +3, and 18mph to 15 mph -3 in -3 mph, so the speed of the river is 3 mph. That would have been a quicker way to solve it XD!

Have a wonderful day!

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<span>The sum of 324, 435, and 546 is 1305. If this number were to be expressed by the base of 7, we would need to figure out what value of exponent would satisfy the requirement. This can be done by setting up an equation where 7 to the power of x must equal 1305. Using logarithms, one can solve for x and find it to be 3.6866853. Thus the sum of the aforementioned numbers, expressed in by the base of 7, is 7^3.6866853.</span>
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What is the square root of 6 divided by root 456 multiplied by 23
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Answer:

0.30263157894493828288

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The ratio of boys to girls in a group is 5:3. If there are 18 more boys than girls, work out how
zmey [24]
<h3>Answer:  72 people total</h3>

45 boys, 27 girls

============================================================

Explanation:

g = number of girls

g+18 = number of boys, since there are 18 more boys than girls

For example, if g = 7, then g+18 = 7+18 = 25. In this example, we have 7 girls and 25 boys. The problem with this example is the ratio of boys to girls is 25:7 which is not equal to 5:3

We could use trial and error until we land on the solution, or we can use algebra as shown in the next section.

-------------------

The ratio of boys to girls is 5:3, meaning

(number of boys)/(number of girls) = 5/3

which turns into

(g+18)/g = 5/3

after we make the proper substitutions

Let's solve for g

(g+18)/g = 5/3

3(g+18) = 5g

3g+54 = 5g

54 = 5g-3g

54 = 2g

2g = 54

g = 54/2

g = 27

There are 27 girls.

This leads to,

g+18 = 27+18 = 45

So there are 45 boys

In total, we have 27+45 = 72 people

-----------------------------

Checking the answer:

The claim is that 45/27 is the same as 5/3

For now assume that claim is true. Set the fractions equal to each other and use cross multiplication like so

45/27 = 5/3

45*3 = 27*5

135 = 135

The third equation has the same thing on both sides, so it's a true equation. This must mean the first equation is true as well. Therefore, 45:27 and 5:3 are the same ratio and the answer is confirmed.

You could also use your calculator to note that

45/27 = 1.667

5/3 = 1.667

both decimal values are approximate.

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Answer:

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added with the miles she warmed up is 17 miles

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