55-50+45-40+35-30+25-20+15-10+5
= 5+5+5+5+5+5
= 5 x 6
=30
55 - 50 +45 - 40 + 35 -30 + 25 - 20 +15 - 10 +5
= 5 + 5 + 5 + 5 + 5 + 5
=5 x 6
=30
55 - 50 +45 - 40 + 35 -30 + 25 - 20 +15 - 10 +5
=(55 - 50) +(45 - 40) + (35 -30) + (25 - 20) +(15 - 10) + 5
= 5 + 5 + 5 + 5 + 5 + 5
= (5 + 5) + (5 + 5) + (5 + 5)
= 10 + 10 + 10
= 30