Algebra Help for Physics: Solving for Dtruck with Speed and Distance

[jKotha]

Hey guys, I'm having an algebra blank, can't figure this out for some reason, I know it simple. aHH!. Ok.. here's the question.

I'm trying to isolate d truck, ie, trying to solve for dtruck from the following.

Dtruck = Dtruck + 100m
75km/hr 88km/hr

I think you multiply 75km/hr to the right side, but there are 2 Dtrucks, and i forget how to manipulate that. Once again, I want only Dtruck on the left side.. so Dtruck = xxxxxx. Thanks for the help.

 

[Doc Al]

Dtruck = Dtruck + 100m
75km/hr 88km/hr

I assume this is meant to be two fractions equal to each other? If so, just cross multipy:
$$\frac{A}{B} = \frac{C}{D}$$
becomes:
$$A \times D = B \times C$$

Once you do that, multiply everything out and isolate Dtruck.

 

[jKotha]

thanks.. but still not totally getting it don't think. If i do that.. I get this right?

(dtruck)(88km/hr) = (dtruck + 110m)(75km/hr)

(dtruck)(88km/hr) = (dtruck)(75km/hr) + (110m)(75km/hr)

Now I'm stuck again.. i feel stupid. I know I should be able to islolate dtruck, but how.

 

[Doc Al]

Subtract (dtruck)(75km/hr) from each side.

 

[jKotha]

but wouldn't that take away the dtruck that i want?

(dtruck)(88km/hr) - (dtruck)(75km/hr)

wouldn't that take away the dtruck?.. and just leave 10km/hr. That can't be right.

 

[Doc Al]

but wouldn't that take away the dtruck that i want?

Nope. Consider this: aX - bX = (a - b)X

 

[jKotha]

Doc, you are a genius. One of a kind. Thank you.. i really appreciate it. I should have already known that.