Elementary Math
posted by Sara on .
This is going to seem like a simple question but how do i solve this without dividing.. 5th grade math mom Question: Lester's car can go 15.4 miles on 1 gallon of gas. How far can he go on 0.7 gallons? Thanks

multiply by 0.7
15.4 * 0.7 = 10.8 
I had her do an exact 15.4 x .7 and came up with 10.78. Just wanted to show her that this was the correct way of doing this problem. She didn't believe me :) Thanks for your help and confirmation

You are correct. Any time you need to divide, you can always multiply by the reciprocal.
That is, dividing by 2 is the same as multiplying by 1/2 (or 0.5)
In this case, I'm surprised that the question of division even arose, since you knew how many miles you could go on 1 gallon of gas. For any other number of gallons, just multiply by that number to get the miles.
Not to confuse the issue, but you can keep the units of measure with the numbers. That is if you want to convert gallons to miles, you need a conversion factor.
in this case we have 15.4 mi/1 gal.
So, to convert gallons to miles, you just multiply by the conversion factor and the units cancel, just like any other factor.
.7 gal * (15.4mi/1gal) = 10.78 mi
You can always multiply by 1 without changing anything. A conversion factor is just a way of writing "1" that includes units. The goal is to make the old units cancel out, leaving the new, desired, units.
So, it's just like converting seconds to hours.
100 sec * (1 hr/3600 sec) = 1/36 hr
.3 hr * (3600 sec/1 hr) = 1080 sec 
I just told my little boy to do 15.4 x .7 and he got 10.78

All I did was 15.4 x .7 which equals 10.78

(4.3×6.2)( ×1.1)=4.3×(6.21.1)