Thread: Need help simple calculation

Will drive for three days. Evey day he drives twice as many miles as the day before. After three days he has driven a total of 630 miles. How many miles did he drive on the first day?
A. 30 mi
B. 90 mi
C. 158 mi
D. 315 mi

Please tell me what could be the right answer.
I chose C. but it was wrong.

Originally Posted by zpwnchen

Please tell me what could be the right answer.
I chose C. but it was wrong.

Let x be the number of miles he drove on the first day. Then he drove 2x miles on the second, and 2(2x) = 4x miles on the third. Since he drove 630 miles in total, we have

x + 2x + 4x = 630

now solve for x

Originally Posted by zpwnchen

Please tell me what could be the right answer.
I chose C. but it was wrong.

Let x be the number of miles travelled on the first day.

So he drove 2x miles the second day, and then 2(2x)=4x miles the third day.

So in 3 days, he travelled 630 miles, so it follows that x+2x+4x=630.

Ca you take it from here and solve for x?

EDIT: Jhevon is way too fast for me...haha...(inside joke)...

x is the number of miles on the first day ....
soon the third day it's going to be 4x=630... to solve x= 630/4 = 157.5 (158)
why need to plus all?

Originally Posted by zpwnchen

Please tell me what could be the right answer.
I chose C. but it was wrong.

In addition to Jhevon's answer:

C: 158 mi on the first day - can't be the answer! Because of"Evey day he drives twice as many miles as the day before. So on the second day he drove 158*2 = 316
So after two days he drove 316+158 miles
On the third day he drove 316*2 = 632 miles

158+316+632 = 1106 miles
So D isn't correct either, because on the first day he drove more than 158 miles (and more than 630 miles in total)

Originally Posted by zpwnchen

x is the number of miles on the first day ....
soon the third day it's going to be 4x=630... to solve x= 630/4 = 157.5 (158)
why need to plus all?

Because you travelled 630 miles over a three day period. You need to take into consideration the number of miles travelled each day (because each day depends on the number of miles travelled the first day) when trying to answer the question.

that makes sense! thanks

Originally Posted by zpwnchen

x is the number of miles on the first day ....
soon the third day it's going to be 4x=630... to solve x= 630/4 = 157.5 (158)

Why do you want to calculate x=630/4?

Chris and Jhevon told you to solve

x + 2x + 4x = 630

This is equal to

7x = 630

divide by 7

x = 90

so B is the correct answer

Originally Posted by zpwnchen

x is the number of miles on the first day ....
soon the third day it's going to be 4x=630... to solve x= 630/4 = 157.5 (158)
why need to plus all?

The problem did NOT say that the distance driven on the third day was 630. It said "After three days he has driven a total of 630 miles." 630 was the total driven in all three days and "total" means "add".