Choosing the best strategy - Calculator or Mental Method?

How many minutes are there from 18:55 to 19:20?

 $1003-898$  

 $5.56\times100$  

 $199+267$  

What is half of 158?

 $58\times9$  

Mr Pearce's favour chocolate bar is on sale for 25p. He has £10 to spend.

How many chocolate bars can he buy?

Calculators cost £3.99. How much would six calculators cost?

 $157\times43$  

 $741\div13$