9/10-2/5= first you must get a common denominator
you don't have to change the first fraction but the second one 2/5 multiply the top and bottom number by 2
9/10- 4/10=5/10 is the answer but you can simplify because 5 divides into 10 so 5 goes into 5 1 time and 5 goes into 10 2 times
final answer is 1/2