Well, for 20 you just add them, to get

. For 21, you do a substraction:

. It'd take him quite some time (does this one even have zeros? :)) )
Answer:
67.1
Step-by-step explanation:
You would see if the 3 can go into the seven and seven into three to make it 4, 4 into eight 8 into 6 to make it 7 and then when you get to eight you would round it up into nine to make it 10.
If the common difference, d , is: Positive, the sequence will progress towards infinity (+∞ ) Negative, the sequence will regress towards negative infinity (−∞ )