Tom has a large photo he wants to shrink to wallet-sized. it's width is 20 centimeters and it's len is 30 centimeters. if he wants the width to be 5 centimeters what should the length be?

We can use ratios, in other words, if one part of the photo is changed x times, how many times does the other part change?

We can set up the proportion as :
width 1 : length 1 = width 2 : length 2

20 : 30 = 5 : x

We do inverse multiplication, or multiply the outer with outer and inner with inner (W1 * L2 and L1 * W2)

20*x = 30*5
20x = 150
Divide all by 10
2x = 15
Divide all by 2 to isolate x
x = 15/2
x = 7.5cm