Write the scale so that the units are the same. 1 in : 25 yd.

 1ft : 12in → 12ft : 144in 
1in : 12ft = 1in : 144 in 
{there are 12 inches in 1 foot, so multiply your feet by 12 to get inches} 

1m : 100cm → 15m : 1500cm 
1cm : 15m = 1cm : 1500cm 
{there are 100 centimetre in 1 metre, so multiply your metres by 100 to get centimetres} 

1yd : 3ft → 1yd : 75ft 
1ft : 12in → 75ft : 900in 
1in : 25yd → 1in : 900in 
{there are 3 feet in 1 yard, so multiply your yards by 3 to get feet. THEN, there are 12 inches in 1 foot, so multiply your feet to get inches} 
OR 
1yd : 3ft → 1ft : 12in → 1yd : 36in 
25yd : 900in 
1in : 900in 
{there are 3 feet in 1 yard and 12 inches in 1 foot, so multiply 3 by 12 to get 36 inches in 1 yard. THEN, multiply 25 yards by 36 inches per yard to get 900 inches.}Source(s):Self (Maths teacher)
hopes this helps :) :D :)