This is an IXL activity but in the maths group I am in, we are learning about subtracting and adding decimals. For example, the problem above is 3.9+9.7. You can use the algorithm strategy for adding decimals. .9 and .7 are in the tenths value. Add them up, carry the 1 in .16 to the ones place. 1+3+9 is 13. So our answer is 13.6.