He formula for a trapezoid relates the area a, the two bases, a and b, and the height, h. a=1/2 h(a+b) solve for h.

If you want to isolate h, move all terms to the other side, leaving h behind. 

A=1/2h(a+b) 
2A=1h(a+b) 
2A/a+b=h 

Hope I helped :)

1. 0.3 •4 is the same as 4 groups of 0.32. 7•0.8 is the same as 7 groups of 0.83. it increases by 0.5 every time, so 0.5x4 it increases by 0.9 every time, so 0.9x5. 0.9•12=10.86. 0.47•5=2.35i can finish the next ones in a separate post