修改【20141208】——understand_conis
修改【20141212】——number_sequence_within_ten
修改【20141212】——number_sequence_within_ten
1107 from sunrise
11/7 by Sunrise
by Sunrise 11/7
Merge branch 'multiply-by-tens'
by Sunrise 11/21
by Lipo 12/24
Merge commit 'cc65639cb31fd8d2d2fe1dfbdf69ba2b49a7b809'
Merge branch 'addition_within_20'
add 5 revised exercises from sunrise
add 1 js need to be reviewed
Merge branch 'substraction-within-20'
Merge branch 'halves-and-fourths'
20141209 from sunrise.
change characters names.
Revised by Yuting. 20141224
revise precision error.
exercise revised by sunrise 1225
Merge branch 'order_of_operations'