Which of the following examples is an error of addition by one?

a.5621 + 2356 = 7977
b.6453 + 2345 = 8698
c.4572 + 5324 = 9896
d.2876 + 4123 = 6999

The capulet and montague families love writing. last year, each capulet wrote 444 essays, each montague wrote 666 essays, and both families wrote 100100100 essays in total. this year, each capulet wrote 888 essays, each montague wrote 121212 essays, and both families wrote 200200200 essays in total. how many capulets and montagues are there? choose 1

Can some one me, i’ve been asking for so long

What are “like terms”? why can we only add like terms?