5 boxes of rectangular gifts are 20cm long, 10cm wide and 5cm high. If tied with a ribbon, a 35 cm knot will flow out of the joint. How many decimetres of ribbon does it take to explode this gift box?

(20+ 10)*2+(20+5)*2+( 10+5)*2+35、

=60+50+30+35、

= 175 cm，

= 17.5 decimeter

An unmodified cuboid water tank, with a square bottom of 6 cm and a height of 0.5 m, how many square meters of iron sheets will be used to make this water tank?

6 cm =0.06 m

0.06*0.06+0.06*0.5*2+0.06*0.5*2

=0.0036+0.06+0.06

= 0.1236m2