B.1 Operation with fraction (distribution)

Find the number in X. 

 X/4 - X/7 = 3

Answer

28

Solution

X/4 - X/7 = 3

X × 1/4 - X × 1/7 = 3

According to the distributive principle, this formula is to be

X × (1/4 - 1/7) = 3

1/4 - 1/7 = 3/28

X × 3/28 = 3

Therefore　X = 3 / (3/28) = 28